You are asking people to point their mining hardware to your pool.  If your pool hits a block, that coin goes into your pool's wallet.  Effectively, you are responsible and holding on to 25BTC that belong to the community.

What's to stop you from taking those coins and running?  It's certainly been done before (hence the reference to Nexious).  You are a brand new account.  People want some measure of security that you aren't going to screw them.  Escrowing that 25 coins is a way to provide people with that security.