It is an intended part of the game that the Cup includes teams of different levels. (Even if they decided not to do this intentionally it would still happen to a lesser extent because the number of teams involved a Cup draw would make matching teams difficult.)

It is a bit of a pita that people can try to manipulate a draw by entering the Season with crap players and buying higher quality after the draw is made but its not that much different from people buying a Scout team after league draw is made.

To prevent this the devs would need to block transfers on first day of the Season and that would harm income so I don't see it happening.