I guess they are just trying to look towards another approach of activism. Their idea must be, "We have the big and colourful gay pride parades, why not try the opposite."
I can see what they're trying to do, but honestly, how many people are going to take note of the few people who remain silent for the whole day? Not many.
I'm not sure how successful it actually is in practise, but I have to agree with you, I don't think that silence is the best form of supporting gay pride.
Edit: My previous post was a bit flakey, so I cleaned it up.