Well they claimed they just found it there, so supposedly the last person staying in that room stashed it and forgot about it, and housekeeping failed to replace the towel. Now stoners forget and losealot of stuff, but THAT is generally the one thing they keep up with (or we rather, since I'm once again back in full force).

My theory is, it was their weed all along. They figured, hey if the night man smokes weed, and we "find" some in our room, maybe we can get a free room out of it. And even if he doesn't smoke, it's still the equivalent of finding a bug or food or something that would 100% your room anyways. Just playing both sides I think. They figured a $10 investment in weed would save them $100 on their room. Regardless, their plan worked.