Its a throw back to the early 80s when no one really knew how HIV was spread. Seen as a possible public health crisis, HIV+ foreigners were summarily banned. The Red Cross enacted an equally ridculous exclusion along the same lines from banning men who've had sex with men even if it was just ONE TIME (and there are tons of 12 y/o boys that should be banned then if that's the case).
Despite every other major blood donation program dropping the exclusion, the ARC insists on holding on to it, yet won't ever give a complete answer as to why.