That logic makes no sense to me. You paid a lower fare and that comes with restrictions for you to choose a seat. It doesn't restrict AS in what seat they can assign you.

Ultimately, though, seat assignments are never guaranteed no matter what you paid for your ticket. There are many reasons why airline agents may have to reassign you to another seat. Fortunately it doesn’t happen very often.

Did you ask if another more suitable seat (perhaps another window seat) was available?