Why would the government do something like that? Assuming they actually did some responsible research on the subject (which they would have to if it is ever to be legalized), they would realize that banning high-THC percentages would be pointless. If I have 10% or 30% THC in my weed doesn't matter. If I want to get to a certain level of stonedness, I'll smoke as much weed as it takes to get me there. Plus, it's not like they're going to visit everybody's backyard and test the THC percentage of everybody's crops. If it's ever going to be legalized, it will be because they realize that telling other people what they can and can't put into their bodies is a pointless waste of money, and that putting barriers on what people can consume only tempts people to go for the forbidden fruit. I don't think they'd spend all that money just to get rid of weed with 20% THC.