Vermillion City is supposed to be kind of a shady neighborhood since it's still developing, so maybe it's a scam. The abundance of Gamblers in the park and how Surge's gym is full of roughneck types seems to suggest that.
Alternatively the bike was always intended to be given away as a prize for something, with the Bike shop partnering with other groups like the Pokemon Fan Club to promote it, and the Fan Club manager kind of screwed the game by giving the winning voucher away.
The bike is evidently super nice since the Cycling Road Bikers and people on the Routes east of Fuchsia try to steal it from Red.
