Well some people will "sign a rage declaration" against the defenders, so they aren't taking the risk and signing up for defense, especially when they aren't going to receive anything.
I think that's true if a faction signs up and is a majority of the defense. Unless relations are already tense, I don't think it's an issue. I've signed up for battles on attackers and some of my faction has signed up for NPC defenses. And I've seen this many times with other factions too.
If a few people from a faction show up to defend a village I highly doubt the attackers will see it as an act of aggression...most of the time people are just wanting to fight in strat battles. And early in the game the village defense has better equipment than the attackers lots of times.
I don't think people getting paid has any effect on people showing up for village defenses. I think the other, more likely, reasons are that they are allied/friendly with the attackers, they don't want to piss off the attackers (if relations are rocky), the battle is at a shitty time, or people are busy playing other things (SKYRIM, HELLO!)