Why would chicken have significant regional differences in flavor?
US poultry farms tend to use a TON of antibiotics... The antibiotics are supposed to stop diseases that would otherwise be rampant in chickens that are housed in close living quarters.  (I might add that the USDA swears that it won't approve chickens treated with hormones, but I've seen enough conflicting complaints that I think it's still worth mentioning) 

To your question, it's not (IMO) that all US chickens are less full-flavored, it's that your typical normal chicken in the meat case is quite likely been raised on the aforementioned hormones and antibiotics... I've had plenty of free-range (raised in the US) and they are noticably more flavorful than the average... 

If you are up for some fun watching, Food Inc repeats a lot of what I've commonly heard about several various US food industries (including chicken): http://www.imdb.com/title/tt1286537/

If you're looking for a true compare and contrast, I'd suggest you buy an "average" chicken and then buy a free-range chicken... You should recognize the free-range version as the same as his international bretheren.

EDIT

As a fair shot for the opposing side, here's a good article from NC State saying why hormones are illegal and pointless to use... That being said, Food Inc showed some pretty mutated birds that were bred for nothing but breast meat:  http://www.ces.ncsu.edu/depts/poulsci/newsletter/newsletter_nov04.pdf