I'm not sure that anyone can answer the "how/why can this happen?" question, since company culture and the reasons and restrictions on stuff like a game room are as variable as the company's themselves.
The best way to get an answer is to start asking w/in your company. Here's how I'd go about that:
- Talk to your direct manager face to face and in private.
Try to keep any defensiveness out of your tone/language - I suspect I'd be feeling pretty defensive if I were you, but try to defuse the situation by being relaxed and at least listening. At the very last ask him where his email was coming from? Presumably you and your fellow employees have been playing 20 minute games for a long time, has that always been an issue? If this rule is really because of a few out-of-line employees - would there be an opportunity for mentoring those few and then returning to a more relaxed rule? And where are the guidelines posted?
This could go one of two ways. Ideally, your manager is a reasonable person, he won't name names, but he'll be able to say exactly what the inspiration was, and maybe concede to easing this rule. Conversely (unfortunately) he could be someone who doesn't ascribe to a relaxed work environment and this was just a good excuse to squash the fun. If this manager is someone you can work with, do so - if you and the manager can work together to gently guide the new folks into a better balance, then you can probably turn the status quo in your favor with no hard feelings. Be ready to step up, though, and offer to chime in when you see others abusing the privilege - don't put it all on the manager, if the culture wants to stay relaxed, it has to have everyone consenting to the norm.
- If that fails - take it up a notch.
Ideally, find a manager that is at the union of your management structure and the folks to who still play games before 5:00 - that person may well be able to give you some clue on what's going on here, and some insight into how to change things back to a more company-wide balance. At the very least, this person may know why there are different levels of permission here.
At this level, it's a whole lot less controversial to ask a question than demand a change - and often it will demonstrate your point. "Why can XYZ team play ping pong before 5:00 but our team (ABC team) can't?" will make the person really engage and think, instead of putting them on the defensive immediately.
- Ask HR or another public person/group responsible for company policy and morale. For example - some companies have an ask the CEO.
Realize that the more publicly you ask, the more generic and political the answer will be. But also it works as a last resort... you really want to get #1 and #2 attempted first, as if your manager is not going with company codes, this would be a loss of face for that manager.
But these are probably the people most in control of the policy and changing it.
I'd be very careful to only take this as far as you need to in order to find an explanation you agree with, or a change in the rules. After you win, stop pushing. ;) It's not a survey.