Firstly, gambling is illegal for minors. Secondly, have you ever seen slot machines? Players like to pick machines they feel comfortable with. This is Zynga using their own branding to diversify into another product.
You think making something illegal stops it from ever occurring?
Secondly, have you ever seen slot machines?
Honestly, no, although I've seen fruit machines in pubs, which I presume operate on the same principle; lots of flashing lights and characterization.
I would be extremely surprised to see something branded like the FarmVille page.