We give away a TON of money in bonuses, free $, etc.
The reason companies that legally do business in the US do this (e.g., King.com), have policies like this is that if we have no policy, then at some point the state gets this money. It's called escheating, and we don't want to get escheated by the state!
So all you have to do is either withdraw or play 1 game every 4 months.
Note: we've never charged this inactivity fee, but at some point we'll have to for accounts that are 100% inactive for a long time.