why is it against the rules to give someone under 20 qi tc through decapprize

It defies the whole purpose of prohibiting trades with users under 20 qi.

So long story short: it helps prevent nefarious behaviors with scammers and alts.

it takes like a few minutes get yellow belt
and it still prohibits item trading

Regardless of what you believe, it keeps people from breaking the rules in other ways. Like you said, just get more qi and then do trade.