as far as athiests are concerned, it logically doesn't matter what you do to a dead person, it is simply irrelevant. once you are dead, you are just a rotting pile of meat, bone and "juices". you could flush the body of an athiest down a giant toilet, or recycle it into fertilizer, for that matter. but culturally it is in bad taste or whatever. what one religion does to an athiests' remains is their problem, so if you go around making dead athiests into honorary members of heaven's gate, that is just a bunch of useless fluff.
however, if you have one religious group does to another religious group, without their permission, and against their own laws is something could be considered "intending to cause a riot", or "hate crimes" these days. the logic of the posthumous conversion may be sound, when thinking of the concerns of *the individual who is dead*, but it can be considered offensive to the surviving members.
as a matter of fact, i have some close Mormon relatives. i have explicitly requested that they do not do a "baptism of the dead" on me after i'm gone. even though i don't think it has any affect on me at all, i wanted to state my case that i thought it was offensive to me, in principle, to do anything to me without permission.