[esafak]

Users should not be allowed to select their treatments; it defeats randomization, which is what allows causal inference.

[mandelbrotwurst]

Sure, they'll be more predictive that way, and simultaneously it's valuable to not piss off your customers.

[esafak]

In that case I would take them out of the experiment and impute the censored data.

https://en.wikipedia.org/wiki/Censoring_(statistics)