Completely disagree. On a number of counts.
First, his issue goes beyond an "antiquated, out-of-date marijuana policy". First, use of the drug recreationally is illegal in the overwhelming majority of the states in the country. Having a policy against the use of a drug that brings criminal charges if busted is not a problem. In fact, it's the right thing to do. The NFL's policy allows you to fail three tests before it impacts a player financially, and four before it impacts playing time - re: eligibility. The reasoning is to create programs to rehab players without immediately casting them aside. It certainly isn't the industry standard around the country to be lenient for failures of drug tests. At the places where I work/have worked, if I failed the drug test coming in, I would not have been hired.
The larger issue is not something as narrow as the policy. The issue with Gregory is one of an inability to follow rules and an inability to discipline himself. The policy, at its core, is a simple rule of the workplace. In the NFL, so long as you aren't in the system, you are subject to one test per year, which is done during the time between the draft and the first minicamp. This test is known as the Idiot Test. You literally have to stay clean for one month, then you can smoke up all of Jamaica once you have taken the test. All you have to do is be disciplined, and it can be done. Gregory couldn't do this before he was drafted, and he didn't have money at stake. What would make anyone think that he would suddenly end up disciplined when he wasn't before he got the paycheck?
If you can't follow the rules, you end up firing yourself.