Let your users know because:

1) Optimizely's bad process is an opportunity for you to have something better. "All our competitors trick you so that's usual and don't be mad at us" kind of attitude is quite mediocre. 2) Showing that you care about customers would improve brand loyalty thus keep users and would benefit you in the long run, than a couple of tricky financial profits 3) It's the nicer thing to do and you'd keep your integrity.