The problem is that many times the data collected is not objective at all. Statistics can be twisted by even just the definitions of words used. For example, a CDC study on rape considered forced penetration, but not being forced to penetrate, as rape. This data was then used in a report that, among other conclusions, concluded that women almost never rape men. All the cases of women forcing a man to have sex were defined as not being rape, twisting the results of the study.
When you see someone against data collection, are they against using good objective data, or do they have reason to believe the data won't be objective and manipulated to push one side or the other?
All this being said, I think drugs like MDMA should be legal for recreational use, much less therapeutic use.