Short answer:
To prevent players from withdrawing large sums from a free bonus.
Long answer:
Casinos set max-cashout limits to manage risk. If they let players withdraw unlimited money from a free chip, the casino would lose money. Limits keep bonuses sustainable. For example, a $25 free chip might have a $100 max withdrawal. Anything above that goes away, and you withdraw the capped amount. This is standard across almost all casinos.