Chameitz on Erev Pesach - issur lav?

The Rambam (1:8) writes that in addition to the mitzva of tashbisu, there is an additional lav of "lo tochal alav chamietz" (do not eat chameitz with the korban pesach) that applies after chatzos on 14 Nissan during the timeframe the k.p. was offered. The Ra'avad comments that it specifically says by bal yera'eh ubal yimatzei the words "shivas yamim" -- they only apply during the actual seven days of Pesach, not Erev Pesach. It is hard to understand this hasaga. The Rambam only suggested that the lav of "lo tochal" applies, not bal yera'eh! Apparently the Ra'avad assumed that if there is an issur lav on eating chameitz, there must also be bal yera'eh. Why should this be the case and does the Rambam agree?
Once again, no time for a full answer now - I hope I am accomplishing something by at least raising the issues (she'eilat chacham chatzi tshuvah). I get depressed over the time pressure of dealing with job, family, and trying to learn something, but that's life.