Ok, for the sake of argument here I am going to count all game stores (Gamestop, GAME, EB, HMV, etc) as similar because from what I hear they are, I'm probably going to refer to GAME because thats where I work.
I had a woman come in and buy GTA on the PSP for a 7 year old a few weeks ago, a 7 year old! Did I think 'This may not be suitable for this child but its not my responsibility to comment, the parent must have done their research on the game.' Did I hell, I did everything in my power to try and talk her out of it I spent a few minutes calmly explaining the content and one of my co-workers even joined in and advised her against buying it and she got said it was fine and walked off with her son clutching his new game. A few days later I got a call from head office, a complaint had been put in about me from the very same woman because when she saw her son playing the game and kicked off when he saw him running people over indiscriminately. Now I'll admit thats a pretty extreme case but does that not give you some idea of how some people don't research the game and don't even listen when they are told what it is like. Also you'd be surprised how many people say, 'oh it's fine' a few times but then when you get the point across suddenly change their mind thats happened to me many times. The amount of sway a kid has when they want something is staggering and parents sometimes need a kick up the arse to say no, even if they want to.
Also the standard line from some of the younger people in this thread seems to be 'Some people my age may not be mature enough to play this game but I am.' But how does the shop assistant know that? How is he meant to tell the difference between one 16 year old and another and the intricacies of their personalities just by looking at them. I know it's not terribly fair, but when dealing with age ratings you have to treat everyone like they are the slowest, least developed members of society, because the fact is, when you do come up against someone who is 16, trying to buy a 17 game and aren't developed enough to handle it and you think its ok because 'their only one year off' you can get into serious shit for it, the aforementioned £5000 fine, loosing your job and possibly even jail time.
The assistant you dealt with was a bit of a douche, sure but he was doing his job, informing parents of the age ratings on a game, not deciding you couldn't buy the game, but informing your mum about the content, I don't really see how it hurt and I say good on him. Also as I understand it, the reason you Americans (or it may just be California, forgive me for not being sure) are all up in arms about game ratings is because other forms of media like say films, are self regulated by the industries and games are not, leading to all the trouble about making them legally binding (which they already are over here and it is a great system, but thats a different issue.) But as soon as someone does make a step towards trying to regulate, everyone bitches about it. After all, if you went into a video shop with your mum at 17 and got her to try and buy Hostel or Alien for you, the answer would be a resounding no.
One last thing you also have to remember, which yes, is completely hypocritical considering what I just said and yes, I am ok with that is this. People that work in retail do not like you and yes I ID people all the time just because I can, and yes I'll make a customers life as difficult as possible if their shitty with me. So a lot of it will have been dependent on how you guys acted as well, did you walk up and smile and say hi or did you guys just slam it down on the desk and start talking amongst yourselves, when he tried to explain the content, did your mum say 'yes thanks I'm aware of the content and I'm ok with it' or did she go 'it's fine' in a really annoyed voice as so many people do. Because I come in here and see every other person moaning about and belittling people that work in shops but then also expecting their tongue up your arse, you can't have the best of both worlds you know.