Keith is right that marijuana legalization would create a new player in the political pulling and hauling over marijuana policy: the newly legal industry. The industry would, as he notes, have a strong interest in opposing rules that would reduce the volume of sales: in particular, to oppose any effective steps to discourage heavy use or to prevent use by minors.
That’s not quite the same as saying that the industry would oppose all regulation: incumbent firms would be expected to try to use regulations to stifle competition by preventing new entrants from starting up.
We already have an example of this: the industry that has grown up around the purportedly medical use of marijuana has become a brake on the political movement for legalizing production and sale for explicitly non-medical use. There’s nothing the “medical marijuana” operators – both growers and retailers – want less than the price collapse that would come with full legalization.