Because he's only doing his sworn duty? I don't want a cop who only enforces the law's he wants to enforce. I don't want a President who only enforces the law's he wants to enforce. The best reason I can give you as to why I believe this is to read Kant's "What is Enlightenment." It strikes me as a perversion and a corruption of the government to allow private interests to corrupt the public's desires.
Here's a weird argument: since Obama is stringently enforcing the law, he's pissing people off, and at the same time making us realize what a giant waste of resources the entire ordeal is. By actually enforcing the laws that are on the books, it allows people to realize how bullshit some of those laws are, which will turn into their repeal. If Obama wasn't enforcing the law, it would simply create a bigger industry, and then come the next Republican president, and you'd see a crackdown which would have even worse consequences, and we'd be stuck with the law people forgot was actually in place, becuase we forgot it was there, and never did anything about it.
Now, I agree it's a waste of resources, but unless Obama dismantles the entire War on Drugs, we're already talking about a waste of resources, and Obama doesn't have that power. What we're really talking about is how to direct resources that are already wasted. I mean, if they weren't going after the marijuana industry, what would the DEA target... crack, cocaine, lsd, ecstasy? Is that really a better use of resources, or just a different way of wasting them? Or if it's porn... do you really consider that a better use of resources? I'd rather dismantle the DEA, and stop the waste of resources from ever happening.