'No Overnight Guests" rule on rental postings?

[Jon K.]

I find this "no overnight guests" listing on rental postings interesting.

Granted, most of these ads are for roommates, not single-family dwellings.
Still, it brings me to a few questions.

1) If you've had roommates, do you have a "no overnight guests" rule?
Is it to avoid people bringing home hook-ups from bars? Is it to avoid a boyfriend/girlfriend moving in? Is it to stay under the occupancy limit, avoid more utilities, have more space for paying roommates? If you don't have a rule, do you not care who else stays at your dwelling so long as someone pays? In college houses, my experience is that people usually didn't care who slept in an apartment, so long as someone paid for each room. I'm guessing people with the 'no overnight guest' rule are older.

2) If you rent out single-family units, can you even have such a rule? And, why would you? Is it to make sure no non-listed tenants move in? Is it just a rule on paper to possibly avoid some liability for the additional person, without enforcing the occupancy rule? I know more people living in a property can cause more wear/tear, utilities, etc. But, is such a rule allowed and why?

What if you have a person "living" / sleeping there during the day? Someone with a night job could still move in and skirt the no overnight rule.

I've heard of a "max occupancy" rule where someone staying over 2 weeks straight has to be on the lease and pay rent. That makes more sense.