Some pools are willing to manually cash you out if requested, but a cash out button would allow others to cash you out anytime they felt like it since your stats page isn't private.  The only way it would be feasible is if the pool made you register and you had to log into your account to see your stats, then a cash out button wouldn't be a problem, however most people don't want to register like that.  Mining in a pool is basically completely anonymous.