This is a standard policy for
poker sites. It helps prevent money laundering and keeps them from becoming a free money broker.
For example, lets say that your buddy deposits €250 (250Euros) via credit card and gets $369 in American dollars. PokerStars has to pay the credit card company maybe $4 for the transaction in fees but your buddy is getting full value for his deposit (€250 = $359).
Now your buddy transfers the $359 to you, and you try to cash out via check in American dollars. You are totally bypassing the currency conversion fees normally charged by banks and are costing Pokerstars another couple dollars in labor to make out a check and mail it to you.
Plus Pokerstars is taking a risk that you could be using them as a money laundering facility (to mask drug deals or whatever), which is likely to incur harsh consequences from governments (being shut down, having their funds seized, criminal prosecution, etc.)
So to keep themselves from losing money and to prevent money landering they make you play with the money.
I *think* you just have to make $350 in bets to unlock the money, but I could be wrong. It's best to email
support@pokerstars.com and ask them.