Same reason people go to restaurants vs cook at home.

I just found out that a rental property I own is within 200 feet of a potential new site for a fast food restaurant.

And as a local, late-30's area resident, looking for restaurants, bars, nightlife, shopping, thing-to-do, sight see...

Usually it's some kind of Lowe's gift card or similar, stuck somewhere in my closing package, or maybe a restaurant gift card.

I have no idea how many units the person had (I was straight up eavesdropping at a restaurant), but it just sounded like unnecessary work for units that will probably get rented out regardless. 

If your neighbors don't like your Airbnb, you'll get shut down by code enforcement.  

proper notice to enter is usually defined by the state as 24 hours notice. some states require 48. that is your starting point. that being said, coming through with a high amount of showings can be considered as disrupting the tenants right to peaceful enjoyment of their home. if you plan on coming through with one or two, you should be ok (legally) to give tenant prior written notice and come in regardless of their objections. if you think it may take multiple showings, you are likely better off to work with the tenant to set up a block of time for an open house. in extreme situations, you can consider offering a gift card to a local restaurant or movie theater to get them out. the tricky part is striking a balance between what is legal, entering with proper notice, and what is fair and reasonable, since the tenant can purposely leave the house an extreme mess to make it harder to re-rent, badmouth you to potential renters, and may even try to refuse entry. how you approach will depend on your relationship with tenant.

I wonder what regulations people have run into and if a city/county has ever shut someone down for dividing the two spaces and renting them separately. 

I went to a restaurant that had AC set at low 70s but it was 88 degrees because the AC could not keep up.  

and then I shut up.