It is a bit of a gray area as her car is in the parking lot which is company property however it is rather rare for an employer to do this. The sticker would usually have to be an egregious violation, something that would reflect badly on the company as a whole.
There is ample support for saying that you may not wear political clothing at work but a bumper sticker? I would say that there is a lot more to the story then is being told.
Her comment, "You can't tell me who to vote for!" is very odd. How does that leap happen?
I'm a Vietnam Vet. To me a Kerry sticker, displayed on my property where the public can see it is egregious.
I'll give you a for instance. One of my son's used to have a Seattle Mariner's bumper sticker on his car. I asked him to remove it or not park in our drive way.
Why? Because the Seattle Mariners is the only major league baseball team who refuses to fly an American flag anywhere on the outside of the building or where it can be observed on the outside of the building. They had one displayed for two days then removed it because of public complaints (this is Seattle).
If the Mariner's cannot display out flag I will not allow anything supporting them to be displayed on my property. When I explained to this my son he removed the sticker and he and his wife sold their season tickets (he did send a letter to the Mariner's management first and was blown of).
My property...it's pretty simply.