That's because turtles were banned as pets in 1975, and turtles, like all amphibians and reptiles, apparently, carry salmonella bacteria and were a major cause of salmonella cases--and even deaths--among infants and children.
Coming a bit late to the story, the NYT reports that pet stores in Wyoming and Wisconsin have been selling or giving away the little creatures in defiance of the ban.
Wyoming? Wisconsin? They've been selling turtles from bowls on the street in Chinatown since before I first moved there in the early 90's. Just a few weeks ago, in fact, at the McDonald's on Canal St, I held the door open for a mother with a stroller who asked where she could buy her 2.5 year old son (Jake, of course) some turtles.
I told her that 1) they were diseased, and 2) the turtle vendors aren't out today because it was like -10 degrees. She was not deterred.
Tiny Pet Turtles Return; Salmonella Does, Too [nyt]
Salmonellosis Associated with Pet Turtles --- Wisconsin and Wyoming, 2004 [cdc.gov morbitity and mortality weekly report, sounds like a real pageturner]
McDonald's, Canal & Lafayette, in Chinatown - YES [DT: nyc men's room changing tables]