In Anya and Tara's case that was it at the beginning. Later on, they got motivated.
As for Oz, we'd seen him in The Wish...
Also in Surprise he was shocked that vamps are real, but got over it pretty soon and was ready to fight. It wasnt because of Will, he'd have done the same in any case. Wonder what'd have happened if Will actually got turned in Doppelgangland. Would he have killed her?
Tara could have turned out either way.
She was scared and abused and she was ready to do everything to fit in.
She sabotaged Will's spell, lied, then did a harmful spell to hide her identity that endangered others. Good thing she'd got accepted by Buffy and supported by everyone (even Spike) in Family or she may have turned bad like Faith. Say, if her relatives dragged her back and abused her and maybe she'd killed accidentally some of her gross relatives...
One wonders what would've happened if Faith got that kind of support in s3 Tara got later.