When out on my motorbike in the summer months, I've had this done to me loads of times.
I think as there is a propotion of drivers who are oblivious to what's going on around them and so do not soak you with any malice. In this scenario, they are genuinely cleaning there windscreen and I'd rather they be able to see where they are going.
There are a small moronic minority who will spray you deliberately, thinking it's funny. It may well be you've just discovered one of these mugs.
Chalk it up to experience and I hope you're not bothered by this sort of behaviour again.
Si