They can. It just doesn't do anything for them nutritionally. I think Spike likes trying to gross people out with all the things he can dip into his blood. Xander wouldn't eat twinkies for weeks because he walked in on Spike dipping twinkies in his blood. I don't think it's so much a vampire thing as it's just the people we come into contact with and they can withstand my/our lifestyle. Normal people are in a lot more danger.

She did actually. She cut Dawn, just shallow cuts. When the blood stopped, portal closed. She wanted it open a long time so that she could tell this world apart in the process of going 'home'. By the time I got to her it was too late. The portal had been open too long and the world was being ripped apart. There was a dragon circling Sunnydale. Dawn's made from a part of me. Dawn's blood is my blood. I kissed her, told her I loved her and I jumped into the portal. It closed, dragging all the nasties with it and I ended up dead. For four months before Willow brought me back. If you call me a Zombie I'll bap you.

Yeah it is.