I think steliosh is talking about the motor that comes with the B2 EZRun combo:
Incidentally, the B1 combo is dearer at the moment. I asked them about that yesterday. I'm fairly sure it's just a mistake on the pricing.
There are two types of 540 sized EZRun combo, with just a different Kv motor for B1, B2 and B3 (380 core in a 540 can), but a bigger ESC and more powerful type of motor (full brushless 540) for the B4 and B5. It looks like someone accidentally listed the B1 with the B5 price - in fact, if you bought the three parts of the combo on their own, they'd be cheaper than the combo price for that one. I expect they'll fix it soon.
I'm not really sure why the B5 is dearer than the B4, because the motors in those combos cost the same, and are the same spec, but with just a
different number of turns:
I'm no expert on bikes, but I expect steliosh is right about the amount of power needed. The 6000Kv motor might be so fast is strips the gearbox, even on a 2s lipo. They don't give current ratings for the motors but if you actually got anywhere close to the 60A continuous the ESC is rated for, even on a 2s lipo, at say 7V, that's 420W. One horse power is 746W, so that seems a bit excessive for such a small bike. I'm sure someone has tried it though