what is it that detemines whether buildings in a friend's town are tappable or not?

It usually means that they haven't cleared their town (during an event) or they are no longer playing. You can check each player in the friends list screen by tapping on the icon next to their name to find out when they last played. Not all buildings can be tapped durning this event.
When there is no event on, it is a good idea to leave some buildings untapped for when friends visit.

it can't be that they're no longer playing- i have friends who haven't played in years who still have tappable buildings every time i check their town, during events and otherwise.

.... sooo..... anybody???

Your neighbors who haven't played in a long time are reset with each event. You may be the only person that still visits them so there is always something to tap because of the reset. I have a couple neighbors who just spend donuts to rush all the crafting and prizes. They never have anything to tap and they never or rarely visit me. (I don't know why I keep them). If this is just one or two neighbors, just get rid of them and get someone else from the add me list.

oh.. i dunno, its like every other person on my list... ive been wondering a building in your town being tappable for friends had something to do with that building being tappable in your own town

In this event, you do not have to worry about leaving your buildings untapped. You can collect all the buildings and other people can still tap as long as you have cleared the pins off. Maybe they are asleep when you are visiting and so haven't cleared them. If you are not attached to them, I would just get some new neighbors.