This is not only an issue for clinical trials, but can also be an issue for any research that will screen out potential participants.  There can be valid reasons to exclude groups of potential participants from research.  It is vital that researchers explain why groups are being excluded.  If that doesn’t happen, there is a real chance some of the excluded persons will take umbrage and complain.  Though perhaps not in quite as funny circumstances as those shown by this Don Mayne cartoon.