It kinda seems like a dumb move, a thinly disguised moneygrab. Anyone providing any sort of online service for money should strive to maximize their compatibility and exposure and reach. Maybe Win9x and IE4 and text-browsers aren't "worth" the effort, but today's customers have alternatives ... if you make things inconvenient or more expensive then you'll lose business to competitors who're trying to support every platform and every device they learn about. To be sure, there are people who'll try to haggle and lowball over your offerings, there's always some business you don't really need or want anyhow, but it's unprofessional to baldly apply such surcharges as a penalty.
I have to wonder which of their marketing geniuses decided to call this a "tax" - nobody wants to ever pay extra taxes for anything, and it's not like this tax is going towards a governed infrastructure. It would've been smarter for them to say "we don't support your sucky old browser, click here for free upgrade".