The reason is mainly technical and the data model we use can provide results for AND very fast but not for OR.
Also I would not know how to include that in the UI without making things complicated. You can select two labels at the same time but how would you specify whether it’s an AND or OR?
I could see at some point offering an advanced search where all these things can be specified including negative searches which are not possible at the moment either (http://forum.paperpile.com/t/negative-searches/40)
But the demand is extremely low and we are extremely vigilant not to introduce complexity for features that don’t really add value to most of the users most of the time. I’m aware that’s not what you want to hear but I guess that’s part of my job 