As I understand it, the allegation made against them is that they are only legal because the manufacturers exploit a loophole in the certification process; they demonstrate that the box itself doesn't emit much interference and say that the interference subsequently emitted by the unshielded mains wiring is outside their responsibility. It's not unlike the situation with diesel engines that don't exceed emissions standards in lab tests but do on the road.
This may change; OFCOM are to update the Wireless Telegraphy Act to alow them to require users of interfering devices such as these to stop using them:
https://www.ofcom.org.uk/__data/assets/pdf_file/0020/55307/annex6.pdf
https://www.ofcom.org.uk/__data/assets/pdf_file/0017/58220/statement.pdf
Their statement points out that, for example, in the past they could take enforcement action against a fluorescent light that was causing interference but not an LED light because that wasn't specifically mentioned in the regulations! Now the rules will apply to any "apparatus" at all.