As I understand it, it doesn't just apply to faulty goods. The law has been changed to bring online sales in line with shop bought goods and to give a fixed time rather than have variations between companies. They need to update their T&Cs in light of the changes in the law, if they haven't done it already.
They are allowed to offer customers better terms than the law specifies but not worse ones.
It also says that the 30 days only start from the day goods are actually delivered - not from when they are ordered - which clarifies things for those occasions when things take an age to be despatched and then take the scenic route to get to you.