As the microgaming network that these casinos operate on (all of the free play casinos with the big bonuses are on microgaming) does not allow US players due to the UIGEA restrictions.

When this gets resolved US players will be allowed to play anywhere.

However, there are plenty more bonus available for US players, all of which are listed on the main page of this site with the handy little tag as to if US players are permitted or not...

This can be found here - Instant no deposit casino bonuses