You have a higher opinion of the decision-making capacities of 7 year olds than I do. I don't see a lot of second-graders making rational decisions about what team or player they want to follow (or rational decisions about much of anything, for that matter.)
You don't think 7yr old kids watch SportsCenter and see all of those LeBron highlights and eventually beg their parents for a LeBron jersey for Christmas? Really?
Hmmm.
I was never a golfer, could care less about the sport. That didn't keep my kid from becoming a HUGE Tiger fan.
My oldest son became a Duke fan because they were always one of the best teams in the NCAA while he was growing up. This even though both his parents HATED Duke.
My youngest became a Michigan fan because of the fab five and still follows them in both football and basketball today, even though he never stepped foot in the state and until last year's Capital One Bowl never saw either team play in person and his mothers beloved Penn State is one of their biggest rivals.
Steve will appreciate this one, or not (LOL!). We have a very good friend who is a big Missouri fan. Her daughter became a Kansas fan at the age of 8 partly because she went to school with a bunch of kids who were Kansas fans. Missouri and Kansas absolutely hate each other and this girl had been going to Missouri games with her Mom who had season tickets for years. But the Tigers weren't winning and the Jayhawks were so she went with the rest of her friends and became a fan of the team her mother despises.
I don't find any of these examples irrational, in fact quite the opposite. Much as you seem to think it's impossible truth is kids of all ages do have minds of their own and, yeah, they tend to like players and teams that are high profile and exciting.