Question: Can a landlord prohitbit firearms on their rental property?

Good find, I hate vague laws, because that could be destroyed in court. (Im just playing devils advocate, it's what I do:))
First, its "void for vagueness" as we cant determine the meaning, ie.
Without a comma after "landlord" it means:
"a landlord of a hotel or motel; OR an operator of a hotel or motel" Implying that it refers ONLY to hotels and motels which are businesses and DOES NOT affect the rights of a NON-business private residence being rented in a private capacity.

In order for it to apply to anything other than Hotels and Motels (Inns, Lodges, Bed & Breakfasts are included by the inclusion rule of law) it would have to be written this way: (notice the commas)
Section 6. Landlords and tenants -- no firearm prohibition allowed. A landlord of a property, or operator of a hotel or motel, may not, by contract or otherwise, prevent a tenant or a guest of a tenant from possessing on the premises a firearm that it is legal for the tenant or guest to possess. A landlord of a property, or operator of a hotel or motel, may prohibit the discharge of a firearm on the premises except in self-defense.

the way it is currently written, I BELIEVE, is to be confusing and to remove property rights...in its current state it would be as confusing as this example:
"A supervisor or a manager of a Mcdonalds or Burger King must wash his hands before returning to work." does this mean ANY supervisor or just one from BK or McD's?

secondly Montana annotated code of 2009 at section 70-24-311 says a "landlord" can "adopt rules" as long as it's for safety AND IT'S in writing:
70-24-311. Landlord authorized to adopt rules. (1) A landlord may adopt a rule concerning the tenant's use and occupancy of the premises. A rule is enforceable against the tenant only if:
(a) its purpose is to promote the convenience, safety, or welfare of the occupants in the premises, preserve the landlord's property from abusive use, or make a fair distribution of services and facilities held out for the tenants generally;
(b) it is reasonably related to the purpose for which it is adopted;
(c) it applies to all occupants in the premises in a fair manner;
(d) it is sufficiently explicit in its prohibition, direction, or limitation of the tenant's conduct to fairly inform the tenant of what the tenant must or must not do to comply;
(e) it is not for the purpose of evading the obligations of the landlord; and
(f) the tenant has notice of it at the time that the tenant enters into the rental agreement or when it is adopted.
(2) A rule adopted by a landlord must be in writing and must be given to each tenant residing on the premises and to each new tenant upon arrival.
(3) If a rule is adopted after a tenant enters into a rental agreement that works a substantial modification of the tenant's bargain, it is not valid until 7 days after written notice to the tenant in the case of a week to week tenancy or 30 days' written notice in the case of tenancies from month to month.

The fact is, when it comes to private property, the OWNER makes the rules, BECAUSE, you dont have to go there, thus it's avoidable, but a Private Property can limit rights, as long as its agreed upon.

You are right that the tenant would ONLY have a right to put the dish on the part of the property that THEY RENT. BUT....If they are in a multi-family building with no porch or balcony that is defined as THEIRS, they could NOT place the dish on the exterior walls or roof or grounds, which they are NOT renting, but merely have the enjoyment of, as those areas are still "owned" by the landlord.

So if a rental unit has no space in which you could place a dish and get service........ that's not the problem of the landlord unless the landlord guaranteed this service.

We used to have a problem all the time with tenants telling us that they COULD get a dish and we had nothing to say about it (based upon the stellar legal advise that they had gotten from the "dish" company salesman). We would tell them that yes, they could..... as long as it sat INSIDE their unit, which was the only part of the building that they were renting...... AND, if we found that HOLES had been drilled in OUR exterior walls or OUR roof, they would be responsible for the damages.

A few tenants didn't ask us first and had quite an expensive lesson ..... thanks to listening to the advice of a "dish" salesman. (no offense intended adam)
_________
A Man's Country Is Not A Certain Area Of Land,
Of Mountains, Rivers, And Woods,
But It Is A Principle......
And Patriotism Is Loyalty To That Principle .
- George William Curtis

statutes that prohibit "verbal" real estate agreements.

between the two people. The property owner sets out the rules. The prospective tenant's job is to decide if he/she can live under those rules, and then decides whether to agree to the contract.

Deciding to SIGN an agreement, and then BREAK the rules in secret.... in any other business, would be called "breach of contract" or "fraud". So the answer you gave in the first place is correct.... he can put it in the lease. Putting a line through that part that one doesn't agree to, would have to be APPROVED by the owner, and then that forms a NEW agreement. However, to AGREE to a set of rules, and then break those rules that you have agreed to, makes you less than honorable in your dealings with others.

_________
A Man's Country Is Not A Certain Area Of Land,
Of Mountains, Rivers, And Woods,
But It Is A Principle......
And Patriotism Is Loyalty To That Principle .
- George William Curtis