The truth is, there's a lot of naivety behind your question. First of all, you need to understand that these deposits on you wallet are made by the casino because it's essentially a casino promotion. It's marketing, and what they're looking for is traffic to their platform—not philanthropy.

We—and I say "we" because several people from our local board have won these prizes—often use the phrase "winning them twice." Winning the contest is only the first step. After that, you still have to meet the wagering requirements, which sometimes include minimum and maximum odds, and then you need your bets to win. In other words, you haven't really won the prize until you've successfully turned it into withdrawable money.
From that point on, you should manage it exactly the same way you would manage your own money. There shouldn't be any difference. In fact, that's one of the biggest benefits of receiving a prize like this as a free bet: it teaches you how to manage bets that many people would never place with their own bankroll.
So no, the idea that "it's free money, so I can afford to lose it" is, in my opinion, a terrible mindset.