I shouldn't wade in, really, but I'm too much of a grammarian (or grammatician) not to.
Because grammar does have a right, a wrong, and a gray area, it must be recognized that some things are gray area. And because language is ultimately imprecise, it seems to me that the litmus test for any grammar rule should be whether it makes a statement clearer or not. In this case, it is clear from LEGOs (or Heinikens, as mentioned above, or Acuras, or anything else where the product can be counted) what the intention is and, I would say, clearer that LEGO bricks. Would that be a brick made of LEGO(s/bricks)? A single brick? And big brick-like blob of melted plastic (probably not)? The tipping point has definitely been passed with this one . . .
. . . which is not to say it is correct. Only that it falls in that area of grammar where strict adherence actually reduces the clarity of the phrase, as with Churchill's famed "This is the sort of nonsense up with which I will not put," or the fact that the split infinitive rule comes from a law of the Latin language. So, this is undoubtedly fuzzy, and becoming fuzzier as more corporatism breeds more genericized trademarks. But it might be one of those rules that is in need of reform after years of debate by men wearing bow ties and tweed jackets (like the insistence of the Queen's English that "the team
are playing well").
Just my $.02. And yes, I enjoy grammar debates. And yes, this makes me a huge nerd . . .