While strictly true, it's also not the whole point. Most laws on the books are almost never enforced, and even then only selectively. For example, violating the terms of service of any software, website, or ISP you use is technically a felony according to some U.S. courts http://boingboing.net/2013/01/16/aarons-law-introduced-w.html
Moreover, in polls somewhere between a quarter and a half of U.S. adults have used pot - what do you think the unemployment and incarceration rates would be if drug laws were actually enforced?
So, the police already have wide discretion. It's a necessity, because the people who write the laws seem to have little interest in writing better ones. The only real question is how the police use that discretion, where they put their (again, of necessity) limited funding.