According to Carl's Guide for TS3, the No visitors allowed means that you don't want random sims visiting that lot.
https://www.carls-sims-3-guide.com/buildedit/edittownlots.php
No Visitors Allowed: Another general community lot type. This one is for lots that you don't want random Sims visiting. An example would be if you build an abandoned house, you would not make it a residential lot as eventually someone would move into through story progression, but if you make it a community lot, you don't want lots of Sims visiting it.
Now why they made 2 lots as No Visitors Allowed is beyond me but you can change that if you want other Sims to show up when your sim is there... :)