I have noticed that some bookmakers will offer free bets but have a condition that in order for the winnings to be withdrawn they must be placed on bets with at least 1/5 odds for example.
If I have a £5 free bet and wagered it on a bet with even odds then I would expect to win half the time but only double my (free) money. If instead I placed the bet on something with odds of 5 to 1, then I would only win one in every five times but then would make a five-fold return. From the bookies perspective they would expect to pay out the same total winnings across all bets anyway.
Why do they care what odds you pick?