I agree, but there is likely a reason.
Has any athlete tried for a medical exemption for meldonium? As far as I can tell, anything it helps can be obtained with legal substances.
Medical exemptions don't apply to every substance, especially known performance enhancers, there has to be a medical designation.
Meldonium does not have such designations. Like you said, it's a supplement...with no medical use. Sharapova took it for something, but did she need to?
For example, it is difficult to get the same results as you can with some of those coritcosteroids with any other drugs. Often, such steroid treatment is down the list, after others are tried.