I suspect this is because technically it would be a PITA to allow users to allow/revoke at their own discretion.
While I agree that it would be nice from the users point of view, the impact of pulling data from the kind of analysis I'd expect them to be doing is going to be a data analysts worse nightmare (i.e. holes in your data set can sporadically appear, so nothing is concrete and all analysis must be reverse-justifiable). If you can reduce the frequency this happens but still give the users the option, this seems like the best of both worlds.
While I agree that it would be nice from the users point of view, the impact of pulling data from the kind of analysis I'd expect them to be doing is going to be a data analysts worse nightmare (i.e. holes in your data set can sporadically appear, so nothing is concrete and all analysis must be reverse-justifiable). If you can reduce the frequency this happens but still give the users the option, this seems like the best of both worlds.