It prevents people from abusing faucet. Like ethan said, someone can claim faucet, invest it, withdraw and repeat. Some people are desperate enough to sit all day milking faucets, and casino faucets are much bigger than classic faucet sites, so owners should monitor faucet activity and develop protection mechanism against such abuse.
Here's another idea - faucet just gets locked for some time (6-12 hours for example) after making investment. This way investors will not be affected and most other users too.
Hey...
Actually not a bad idea!
It will prevent people to milk the faucet in order to invest it! You have to put a rather long duration like 24 hours!
Then people who want to invest won't really care but those who try to abuse the faucet won't be able to invest the money ^^
24 hours faucet duration might be a good idea but its pretty long time to wait for certain people who are trying some strat to use, and maybe I think 1 hour time duration would be rather enough for that one since it abuser cannot do nothing with it since its pretty awfull to wait for that long to earn some little money on faucet.