Your point is kind of moot anyway because humour is one of the most subjective things there is, next to beauty.
There are different degrees of humour designed to appeal to different audiences. There's wit, tragedy, slapstick, timing, ridiculousness, random, observational, stereotypes, surreal, "smart" and many more. You can't expect one person to equally appreciate all types of humour.
While you could argue that certain American Comedy shows cater to a certain demographic and use low brow humour, you can't say that ALL American Shows use low brow humour. There are plenty of well-written American Comedy shows that don't get the same level of appreciation and exposure that Family Guy does. Even still, The Simpsons in their prime managed to use low brow humour among other things in a very intelligent way. It was hailed one of the funniest and well written series of all time, and for good reason.
Does that mean that every single North American finds The Simpsons funny? No. You see my point?