I asked a co-worker who, at one time, managed a hotel here in Minnesota, and he agreed that it's likely to avoid complications about the taxes and because some states grant certain rights based on length of occupancy.

What's interesting is that, at least in Minnesota, he needed a legal reason for eviction, like non-payment, noise complaints, etc. Even if the hotel is overbooked and the guest only made a reservation for 1 night, they can stay as long as they keep paying. After the first month, though, the tax thing kicks in, but they could stay for the rest of their life if they wanted to.