While a daycare should do everything they can to ensure children aren't harmed, you have to realize that this is certainly not a new problem. I know the thought doesn't make it any easier for the bitten, but it is a normal stage which many children go through. I'm sure the parents of the biter feel awful about it, but like you, they also need childcare so they can work and provide for their family.
My daughter who is now 20, was as a toddler, a biter. I was absolutely horror-stricken at the thought that my normally sweet little child was biting others for no reason.. or even any reason. The excellent child care center that my children attended had a rather unusual approach to the 'biter' problem. I was asked to consent to having my child placed in a 'special play group'. If you haven't guessed, this group was of... biters. I was assured that for most children, being in this particular play group was for a very short time only.. usually only a day or two.. just long enough for them to be on the receiving end. As awful as this may sound, it worked and my daughter was 'instantly cured' of her biting ways once she truly understood what if was like being on the receiving end.
At the very least, I feel your provider should remove the offender(s) from the presence of the other children to keep them safe.. whether that means asking them not to return to the center until the problem is resolved (which probably won't really help the 'offender'), or by placing them in a special play group temporarily.