Ok, we're missing a connection here; that doesn't answer the questions of "How?" or address the issues of purity/potency. At this time, they can print pretty much ANYTHING they like on the labels; nothing is yet regulated/standardized, and with "natural products" they aren't required to specify everything that's in a product (goes to purity), much less how much. Along with the higher competition that lowers prices (this is still a "for-profit" business for the most part) comes cost-cutting measures and corner-cutting. Where, and on/with what? Who's watchdogging? (That may change soon, but it's not here yet, and has a way to go—see NPR link below)

It's not like prescription drugs at all. With prescription drugs—pain medications in particular—patients don't generally adjust their own dosage—doctors do, based on accumulated knowledge & experience that isn't yet available with MM.

Putting all that together, there's a lot to question/be skeptical about, especially when the image the industry portrays is one like this:
http://www.caregiverscup.com/
Their hearts may be in the right places, but would you buy medicine from—and/or trust your health to—this image? Are you?

These questions/issues were recently the subject of an NPR report (among others).
Marijuana-Laced Treats Leave Colorado Jonesing For Food-Safety Rules

I think they concern many doctors as well.

Doc