How does this defeat the purpose ? If you could steal any vehicle and own it for virtually nothing nobody would bother buying them and more people would just go hunting for vehicles to steal.
Take for example a Hemmit Box (Chops for 24K ish), Costs (50/60K) to buy new, You pay (10/20k) instead of chopping to keep it. Thats like 34/44K total for the vehicle considering what you would get for chopping it and the amount you would pay. (Still cheaper than the original vehicle price)
This means you can still steal vehicles to get them cheaper but wont discourage people from chopping for money or buying the vehicles from the shops.