and IMGs and DOs (as well as other independent applicants which I believe may include US MDs that have graduated in previous years) are allowed to pre-match. Current year US MD grads used to be able to, but are not allowed this year (I actually met a US grad that had pre-matched at their home institution in a previous year, so those people clearly exist!).
Say a program figures they want you (i.e. you'd be high on their ROL anyhow), there is no downside for them to offer you a slot. That way they've removed some of the uncertainty, especially if they figure that you are a competitive applicant, as another program could extend you their own pre-match offer and snatch you up. Then, if you prematch, the program removes the slot from the Match (hence some of the adjustments in the number of positions available in the Match; I think the deadline for this was January 31, 2003 (Quota Change Deadline)).
I, as the applicant accepting the prematch offer, also withdrew from the Match. For the applicant, pre-matching really only is ideal if you were going to rank the program you pre-match with number 1 anyway, otherwise you're likely to end up playing...what if. In my experience, a lot of programs that take IMGs seems to offer them pre-match positions. By the quota change deadline, when programs were clearly evaluating their applicant pool--as they had to decide how many positions to put into the match, I had gotten 4 pre-match offers including the one I took. Especially at mixed programs, I sometime wonder to what extent a program uses the pre-match process to fill the spots it has decided to "allocate" to independent applicants and then just ranks predominantly AMGs in the match. I think that this may be one of the reasons that the NRMP has tried to change the rules, because it leaves people that have gotten pre-match offers at programs in the middle of their ROL in the quandary of accepting something that was not their preferred option but having a position versus participating in the match and maybe seeing that position gone (I have a friend that is in this position, and has decided to go through the match).
Modifying the rules seems to be their attempt at resolving this problem, although I'm not sure that it really will (at least not for the applicant.) Starting next year, programs that decide to participate in the Match will not be able to extend any pre-match offers to anyone, regardless of their applicant status (i.e. IMG/DO etc.). Some programs that fill entirely outside the match now may not take part in the match at all. But mixed programs will likely opt to take part, in my opinion. It'll be interesting to see what effect the new regulation will have.