From my perspective, it looks like B did not have as good a time as A and decided she wanted to invest her friend-finding energy elsewhere. The unfriending was a "soft no". I tried to explain this to A and suggest she put it behind her and move on. A replied that she thought B was being cowardly and should have just told her that she (B) didn't really want to hang out again outright.
Assuming that A really liked B, but the feeling was definitely not mutual, how should B have handled rejecting A? Do you think A was in the right to call B and ask about the situation?
(Note that this was an entirely platonic meet-up. Neither party was expecting romance or sex.)
There's also the chance that A misunderstood the mechanics of the site entirely and had not been unfriended. If you had met someone online, hung out once and had a good time, but then a week later got a phone call asking about why you unfriended them/why you didn't like them/whatever, when you had actually done nothing of the sort, would that scare you off the person entirely, or would the potential friendship be salvageable?