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Pretty sure I've seen werewolves during the day too in Grahtwood.
Those are not the kind that can infect you. Only werewolved/bloodfiends in The Rift, Bangkorai and Reaper's March can infect players.
Any other WW/vamps (and there are a lot in Glenumbra/Rivenspire) are story related.
ForgottenHeretic, I think it was true in the beginning, and they may still have a better chance of spawning during the right moon phase, but I think ZOS tweaked it because of people camping the spawn sites and killing them.