Earlier quoted context omitted.
Large car rental companies are not exactly cutting edge on IT. They tend to have a lot of small locations that's not exactly easy to start upgrading because of the cost of sending people out to do it all. My local branches look like they had their last refurb in the late 90s for example. One example does not a sample make.
It is not just car rental companies. Most of the UK's major banks use IE6 still. This is because most of their intranet apps will only work in IE6, and they have the size to demand that their supplies support IE6 too. Corporate IT policy won't allow browsers apart from IE. If only Microsoft allowed side-by-side installs of IE6 and a IE8 it might be a bit more bearable (given that IE9 is not available on XP)
How did it ever happen that IE6 ended up in such a long lasting and dominating position? Was it (I am just guessing here) the coincidence of tho things: (1) IE6 just happened to be the most recent Microsoft browser around the time when most of bigcorps initially build their inter/intranet apps, and (2) because this was the first wave of building such things, the web coders did not bother to think about a future when IE6 is no longer a modern browser?