In all the crypto casinos I have played at, free bets can only be used on single bets. And that rule is fine if the free bet is at least $10-$20... but when the free bet is just a few dollars, it's a bit ridiculous to bet on a single game. I am sure most of us would go for a "wild" combo!
I already won a free bet of $25 before which is very good since a 2x odds can give you the $25 back for free and without any risk. And this is not hard to happen since it's a free bet anyways you can just try your luck on a game that you know you have good chances but it's still not 100%.