Agreed with Elles, to me it doesn't seem strict, just sensible. Not allowing sports on the lawns seems no stranger to me than, say, not allowing sports on the lawns at National Trust properties. Firstly, as Elles says, they're lawns - not pitches. Secondly - do you really want people playing a noisy game of football right under your window while you're revising for mods? Trust me, people sitting on the lawn reading the paper and chatting is bad enough by that point - had people been shouting tactics I would probably have gone and kicked somebody where it hurt.
As far as the dress code goes, I don't know about Trinity but for most places gowns = formal hall only. If you don't want to do it, then don't go to formal hall.
The only one that ever got me was the rooms, and as people of said that - like having guests for only a certain number of nights is simply not enforced unless you're making a nuisance of yourselves - and if that's the case then I, for one, would be happy to have the rules.