If you pick a car that is dusty / muddy, you can be pretty sure that the owner is not overly fussy about their car, and hence quite likely to have a positive reaction to a free wash. The probability that someone with an obviously dirty car getting actively upset about an unsolicited wash would seem to be sufficiently low to justify the risk, I would have thought.
Equally though, someone who has a dusty/muddy car (esp in a city rather than rural area) probably values a car wash less than someone who actually has their car washed every week so their car looks relatively clean.
I keep my TT looking clean because I care a lot about the car and so have it washed every week. I'm probably the target customer for a service like this. I'd also be pissed, as I said above, if someone unsolicited washed my car.
I would definitely be very happy if I walked outside and found that my van was cleaned up, even without my knowledge.
The whole "leave your car unlocked so we can clean the interior" thing scares me a bit, though. The audio system in my van is worth $1k+ and it could be easily removed in the time it takes to do a car wash. :I
I was really shocked, I went out and looked at my car multiple times because I thought I must be imagining it. I have no idea if they did it intentionally as a marketing ploy, or if it was like eps's Ferrari comment and someone thought it would be funny to book them to wash my car.
If my car was new and I was still babying it, yeah I might be pissed at first. But the job they did was better than most of the $30-$40 washes I've had, like they actually did a good job cleaning the wheels. And my car was filthy, ever since I moved to SF I stopped washing my car regularly (too much of a hassle and I only drive it once every 2 weeks anyway).
I might still try the service - I currently pay ~$30 with tip at the local car wash place where they wash your car by hand.