Cats are nothing like dogs though. While it's also important to socialize kittens as soon as possible to make them comfortable around humans, I don't think it's a good idea to haul them around town in a car. Cats don't like to be taken outside their known environment at all, and they don't like to be locked inside a box, or transported in a noisy car.

Just to give some perspective, my own cat loves people, strange or familiar, it doesn't matter at all. Take her to the vet in a car though, and she will go mental, crap and pee in the cat carrier, meow and hiss when she gets let outside of it (never does that otherwise) and be extremely edgy for sometimes 2 days afterwards. I wouldn't go as far as saying this is 'cruel' to the kittens, and it's great that the proceedings are donated to an animal shelter, but I'm pretty sure the kittens will not enjoy it nearly as much as the people who get to cuddle with them.