The legalities of this are interesting, a few collegues and I were discussing it earlier. Under Australian law you could potentially argue unfair contract terms under Schedule 2 of the new Australian Consumer Law (2012) and you could get the contract term struck out. However, if Kogan make the argument that the cost of catering to people using old browsers is very high, and the fact that it's so easy to upgrade it is likely that a Court would look favourably towards this policy. Making a penalty clause for the express purpose of punishment is unenforceable at law but on the face of it they're not doing it to punish, only to mitigate the cost of the extra programming.
There's also the issue that if they're promoting Chrome/Firefox/Safari/Opera and not IE while specifically taxing certain version of IE then they may be liable for anti-competitive behaviour, but this is a long shot as Kogan would simply deny they're trying to force users of IE and add IE9 as a choice for download.
There may be some obscure restraint of trade doctrine that I'm aware of (I don't practice in commercial law) but as it stood we agreed that unless the ACCC decided to use its powers to bring an action against Kogan it was unlikely that anything could really be done.