He wasn't busted for growing but for possession of a 1/2 pound which is a big difference.
THerefore there wasn't probable cause that he was growing because he had possesion before.
It's not uncommon for previous criminal to "progress" onto the a higher tier of action once they grew in confidence/experience(pretty much how most serious offenders evolve/devolve) .Its is still logical to assume that someone with a criminal record of possesing marijuana would want to attempt growing them as a next step and the sudden unexplainable rise in power usage could be indicative of that.
Charged with marijuana possession, and yet they confiscate everything bitcoin related.
As somebody already stated:it's not uncommon to law enforcement to seize the asset(cash, gold,gambling chips etc) that they suspected were involved with illegal activities and thing that may contain incriminating data/evidence like computers( it's easy to see how the mining rig comes to play).
This story isn't about a bitcoin mining bust and it never was.The OP should really change the title to be not so misleading.