When you are the heir you take the bad with the good and you generally have no say in the matter. This is what a PU represents in the game-you inherit the crown of the country. The inherit nation in the game represents the permanent assimilation of the new crown into your kingdom in such fashion that it cannot be separated due to different inheritance laws and the like.
Actually heirs can very much abdicate their throne if they want to, it may not be popular with the nobles or the people but it's not unheard of that it happens.
Remember that a Personal Union isn't a direct union between two nations, all it means is that the King holds the title for both nations, other than the head of state the nations are completely seperate entities, with different laws, governments and so forth so it doesn't have anything to do with assimilating the crown into your nation, it's a purely King based thing at least until the King actively merges the national governments.
Even in if an absolute monarch inherit the title of absolute monarch of another nation it doesn't default legally into those two nations being one, they will still be seperate titles, functions, nations and so forth until actively merged.
Even disregarding that, as the holder of the title of King of X, Y and Z, there seem little reason in my eyes why I shouldn't be able to just give away one of those titles to someone, hell I can do that with say releasing vassals which is essentially the same principle, give up my kings claims on certain land that I don't want direct rulership over, I would like a similiar choice of giving up the title/claim to a PU, that is all
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