Forensic Chick, I really doubt there's any law against giving a negative rec. Maybe there've been civil suits where employers have been burned after giving an undeserved negative rec, and so now employers are cautious (my Dad's a lawyer and an employer, and I've heard him talk about this). But in any case, the case law probably wouldn't apply to students getting recs from professors to get into med school.
As far as what to do with your med school LOR's -- schools ask for them in conjunction with their secondary applications. You don't ever send them to AMCAS or to a school that you're not doing the secondary for. If your school has a premedical committee, most medical schools will require you to use the composite letter that the premedecal committee makes up from the LOR's that you have your recommenders submit to them. I think if your school has a committee and you don't use it, you havew to have a pretty good explanation. If your school doesn't have a premed committee, then it is a good idea to utilize some sort of letter-sending service -- most schools have them -- that keeps your letters on file and sends them out when you need them sent (my school did the first 20 mailings, no matter how many letters, for free). If you're not using a committee, then each school will have a different number of letters that they require, and a different distribution i.e. 2 science, 2 non-science or bio (the most common), physics, chem & humanities (only one school that I remember was this picky). Tulane wanted 3 and only 3, no extras, but most schools will accept extras. The most important thing about LOR's is to get them lined up ASAP (like last month!). You are really into crush time now. Best of luck with your application! 🙂