It may actually be costing Kogan money overall by driving away some consumers, but no doubt they've done the sums and think the goodwill from the tech crowd and marketing will outweigh that
This is badly worded - no question. I'm no lawyer, but I imagine there may be some law about actually calling it a "tax" - tax tends to imply something imposed by the government. They are of course free to charge whatever surcharges or discounts they want.....
A smarter move would simply to have notified the customers upon hitting the site that their browser is incompatible with the site and that they could switch to some others that are more compatable. Telling them you are going to charge them more is absurd - either you're going to support Ie7, or you aren't.
Really weird marketing move either way...