who said tables aren't semantic? Tables say "Hey, I'm going to show you some things with a row column relationship now." Some times table is exactly the semantics you need. It shouldn't be your primary and only tool but if your showing a set of numbers collated and sorted by rows and columns then its pretty much semantically a perfect fit.
I see this argument a lot. You're right, there are situations where tables are semantic. But unless otherwise stated, it's fair to assume that someone who's talking about semantic markup knows that semantic markup includes tables, but simply discourages them for 99% of layout purposes.
If by no longer supporting IE6 we can force these companies to finally go with the times, then by all means, no, we should not consider this. We cannot endlessly cater to every outdated technology ever.
Besides, these companies have absolutely zero valid reasons to keep using this crutch - it's severely outdated, a giant gaping security hole and a hindrance to the entire world wide web.
And no, legacy applications built upon IE6 on which some corporations have based their entire internal communication (or other things) are not an argument for supporting IE6, but instead gives us all the more reason to force them to get rid of it.