Powering up for the Eclipse Fight

In Eclipse, Edward mentions that human blood makes vampires stronger, and that Jasper is thinking about cheating on their diet before they go to Seattle (this is before the other plot stuff obviously). As I was reading this, I'm remember later on when they just give Bella bags of blood to drink like a milkshake. I understand Carlisle obviously not wanting to kill people just to power up for a fight, but why wouldn't they just drink donated blood? It's not like it's hard for them to obtain, as long as it's just every once in a while for occasions like this. Come on guys, use yours brains here, almost all of you have drank human blood before in wayy less "acceptable" circumstances. Do you think it was just their egos/principles in the way? Or maybe just a Stephanie Meyer planning hiccup, I suppose