In most cases with avian flu the thing they are scared about is not you getting sick but birds around you catching it. So while birds don't care about borders

In most cases with avian flu the thing they are scared about is not you getting sick but birds around you catching it. So while birds don't care about borders potential sick bird carcass does. Avian flu is not great for farms, if there is a breakout you pretty much cannot have your birds outside. And if your flock is infected they will be gassed and destroyed which no one wants to happen. Hence why they are careful.