This was deliberate because unlike, MTA 0.5 which runs as a separate program, MTA:SA runs as a DLL injected in the game. When you minimise / alt+tab, the game pauses automatically, as does MTA:SA. This means that when you maximise again, you get hit with a lot of packets at once, which at the very best causes strange results and the worst causes a crash.
Believe me, the QA team brought up every little thing possible. Some were postponed for a later release though.