That's by default is the rule of margin trading regardless of where we are trading.

A lot of newbie want to try margin trading for the first time do not know this and when they lose the borrowed funds, they might want to put the blames on Poloniex, so it is good that it is written in the terms.

Just imagine people play with 500X, 2000X in forex trading, and during volatility, their balance even goes negative and as per rule they are replenished by the user

Trading generally is a risky as it is profitable, it can be a burning fire for unexperienced traders and it can be good when traders are experienced and they know when to stop.