This is the problem with today's society. There is no common courtesy in a lot of people. No you don't have to thank drivers for slowing down and letting you cross, but it's polite to give a quick thumbs up or whatever. At the end of the day, it is an inconvenience to slow down and by someone thanking you for it, it makes it more bearable.
Now if she was speeding, then i don't think i would have said thank you either.
It's exactly the same as those traffic calming islands that they put around housing estates where one driver has to give way to another on a straight road. It's common courtesy to give the person waiting at the give way a quick hand palm.