Does it maybe require accepting that random or targeted drug tests can be carried out and you have no right to refuse? Source this is the case in several countries.
Why do/should they care if people in the gym are on drugs or not?
I understand you can be fired from the job because of them (due to negative affect they might have on your work), and when participating in competitive sports, but rejecting someone to the gym for the sole reason of them being on drugs seems in-line to refusing them serving in a supermarket for the same reason.
This is legislation not gym terms and conditions. It is about public policy towards steroids. I.e. one mentally unstable person did something criminal whilst using steroids and then there was a moral panic.
Also, there is no legal basis for subjecting employees or applicants to drug tests. So it does not happen here.
I'd be interested to know which countries you are referring to.