I know if that was in the UK, and an officer saw that tire on a parked vehicle, the owner would get cited for having tire tread below the legal limit.
Over here, it's the same stories mentioned above. People pay $200 in phone bills, $300 for designer heels, $500 a month for a Mercedes they're going to owe money on for the next 15 years, $1,000 for an iPhone...yet something important like tires get neglected. You also have the people who buy the cheapest tires money can buy, but I reckon that's a lot better than not replacing them at all.
In the summer, steel tire belts flying all over the highway isn't an uncommon site. Old tires (at least 5 year old), low tire pressure, excessive heat and what have you.