What's the basis for arresting someone for just being on a watch list? That sounds unconstitutional. I was added to a watch list long ago that results in me b

What's the basis for arresting someone for just being on a watch list? That sounds unconstitutional. I was added to a watch list long ago that results in me being tossed in a cell and "detained" for up to 24 hours every time I go through customs but they always make it exceedingly clear I'm not under arrest (granted while sitting in immigration holding cell it doesn't feel much different). It really gets annoying just being told by CBP "you know what's next" while never being able to find out what list you've been put on. I hope more of these lists become public.