Why do I feel like the whole "must have a certain amount of ETH in your account" is so when they go to hack your account they know it wont be empty?
It could also be so that scammers don't make mass new accts and new myetherwallet address for multiple claims. If they wanted to hack your address they could just look on ethplorer through all the different addresses in every bounty campaign on spreadsheets and get balances anyway. It isn't that easy to hack someones wallet without keyloggers etc so i don't think that is the reason for it.