(rolling probable cause is why)
They could, by the same logic by which they steal (er, "confiscate") goods that are visible in unlocked cars, just tow away locked cars that have visible goods in them that are an "invitation to thieves". Or, for that matter, locked or unlocked cars that, by their value and ease of theft are, themselves, "invitations to thieves".
A guy taking something out of the back/front seat isn't as suspicious as some on going around with a crawbar smashing windows.
I've seen individuals going around a parking lot actively looking for unlocked cars couple of times in my life usually a blow of a horn scares them off.
http://www.newhavenindependent.org/index.php/archives/entry/...
Another interesting tidbit from that: you have to show ID to get your stuff back, and you can only pick it up between 9:30AM and 3:30PM on weekdays. Which is pretty unbelievably shitty.
While there might, somehow, be some arguments that the temporary hold to prevent attracting thieves is neither a deprivation of property without due process (prohibited by the 14th Amendment), nor an unreasonable seizure (prohibited by the 4th Amendment), nor a taking for public use (prohibited by the 5th Amendment) -- though I still think its likely to be at least one of those -- I'm having more trouble seeing even a remote argument for how taking personal property from an unlocked car and selling it, with the proceeds going to the government, can fail to be either a deprivation of property without due process or a taking without just compensation.
Moreover, its exactly the same kind of permanent deprivation of the owners property that the policy is notionally supposed to prevent. "In order to stop your stuff from getting stolen, we had to steal it."
Really?