Was there any other language about smoking other than this from their website (and referred to above)?:

"Guests and visitors to The Cosmopolitan may not consume or possess marijuana or other illegal drugs at any time, including in our rooms, terraces, pool areas, and the casino. This applies to both medicinal and recreational marijuana consumption regardless of the form, including smoking, inhaling, vaping, ingesting, or any other method. In addition, smoking is prohibited in all of our guest rooms and violation of that policy may result in fines or eviction."

If there wasn't any other language, and unless you signed something agreeing to something else, I'd charge it back. The hotel could have banned smoking on its balconies but it didn't; instead it explicitly only banned smoking in its guest rooms. Apparently it went even further and actually provided ashtrays on the balcony. No way they don't know that smoke smell will drift into the rooms when people smoke on their balconies, regardless of the product being smoked.

The terms on their website link the imposition of a fine to smoking in the guest rooms, not to the consumption of marijuana. The quoted language doesn't specify what the punishment is for violating the prohibition on marijuana consumption. In the case of their need to clean due to odors from the marijuana smoke, I think it's irrelevant that you didn't smoke in the room and that they provided ashtrays. The issue there is that you violated one of their rules and they believe they suffered damage as a result. However, unless the law in Nevada provides otherwise, I'd argue that it's up to them to show damages.

Edit to add: Apparently you're lucky they didn't call the police. Misdemeanor with a steep fine.