Isn't there a 48 hour waiting period after switching guilds before you can take part in guild events? This was a thing they put in place specifically to address people guild hopping - or more specifically mercing but it amounts to the same thing. Just saying, what you're describing is not a thing.
My guild values loyalty also, and it's annoying to lose good people, for whatever reason they have for wanting to move on. That being said, generally I wish people good luck and move on with the business of finding a replacement. If they want to leave then they probably weren't a good fit and everyone is better off for the separation. Often, I have people leave and come back - because of that, from time to time we have a waiting list to get into the guild. In fact right now I have a former member that wants to come back cooling their heels in another guild because we're full. Being nice and being kind to people is a winning strategy for keeping your guild roster full.
Anyway, it seems like you're taking this really personally. Seems like an inefficient guild management strategy. Just some food for thought.