Because with this definition of the problem, the "low performers" don't have options to restrict.

My point, as applied to this case, is that giving the option of "smart-only schools" will predictably lead to people taking it, which may lead to issues described by 'habosa here: https://news.ycombinator.com/item?id=13483591. It's a reasonable question to ask if this is what is beneficial to the society (including its economy), and if the answer is no, then not giving such an option in a first place is an entirely logical approach.