Okay..... thank you for sharing stone cold MS company policy. It certainly is very insightful.
Developing a new IE for XP brings in absolutely zero cash (in fact, it probably takes some away from people who might upgrade otherwise) and yet costs real money. People who bought XP have recieved a stonking 10+ years of free upgrades and fixes - that's pretty much unprecedented in the world of computing (especially when you consider that SP2 was pretty much a totally new OS release).
Sooner or later it becomes necessary for those consumers to pay back into the ecosystem by purchasing a new version, rather than just continually expecting something for nothing. The developers of websites, in this particular case, have decided that it is uneconomic for them to produce IE8 compatible websites and thus they, not Microsoft, are the ones who have determined that the segment of the market unprepared to upgrade their browser are not economically viable to support.