"While the Department of Justice has not reviewed the specific legislative proposal for licensing and regulating medical marijuana that you indicate is being finalized, the Department has stated on many occasions that Congress placed marijuana in Schedule I of the Controlled Substances Act and, as such, growing, distributing, and possessing marijuana in any capacity, other than as part of a federally authorized research program, is a violation of federal law regardless of state laws that purport to permit such activities," Cotter wrote.

And that, Crichton said, is where the seemingly arbitrary nature of federal prosecution rears its head.

"So, it's all illegal, they're saying, but it's not all going to be prosecuted," he said. "That's a very difficult playing field for people to navigate."

In fact, John Masterson of Montana NORML, which works to reform marijuana laws, said he flatly tells people interested in the medical marijuana business to stand aside for now.

"If people ask me now how to start a business, I just tell them to stay the hell away," Masterson said. "There's simply no way that you could go into business with any degree of certainty about what the federal government might do."