The trouble with being able to capture raiders when you've reached a population limit is that the game is also hard coded so the maximum recruitment difficulty is 99% so that it's never impossible to recruit someone, this way if you get unlucky with a difficult prisoner early in the game you're not entirely screwed by getting a series of impossible prisoners. This also allows players who are lucky enough to incapacitate a raider after they've hit the upper limit of their population cap to be able to recruit their prisoners EVENTUALLY (the caps are soft, technically it's unlimited it just gets substantially more difficult to increase population due to the death rate raising and recruit rate dropping)
Similarly if hard limits were imposed and recruitment was literally impossible, the player might not be aware and could waste valuable time and resources attempting to recruit without realizing their efforts were futile. Short of breaking the immersion and literally telling the player "you can no longer recruit any new colonists" there wouldn't be another way to let the player know that such limits existed or had been reached, which is one reason the current system of "hard but not impossible" is so useful. I do agree that it puts a strain on organ harvesting and surgery practice, though, but by this point in the game the player should be able to reasonably purchase organs for a stockpile.
Similarly if hard limits were imposed and recruitment was literally impossible, the player might not be aware and could waste valuable time and resources attempting to recruit without realizing their efforts were futile. Short of breaking the immersion and literally telling the player "you can no longer recruit any new colonists" there wouldn't be another way to let the player know that such limits existed or had been reached, which is one reason the current system of "hard but not impossible" is so useful. I do agree that it puts a strain on organ harvesting and surgery practice, though, but by this point in the game the player should be able to reasonably purchase organs for a stockpile.