It's tool syndrome. People don't know how to properly weigh the significance of tools, they see an 80 or a 70 on a guy's speed tool and all of a sudden he's getting ranked highly because he has a true "impact" tool. Obviously everyone understands that hit and hit/power are probably more important than speed, but I think when it comes to guys with plus-plus speed, people let the hypothetical upside blind them too much against the probable downside. I'd probably rather have a guy with a 50 hit tool and a 40 run, as opposed to a guy with a 40 hit tool and a 70 run (all else being roughly equal).
That .400 mark could very well be true talent. Minor league BABIP isn't the same as major league. If a guy can run or square up a baseball, then they'll probably BABIP .350+. If a guy can do both, then they'll probably push .400+ as a true minor league talent level. Look at what Trout's were.