OFCOM & Enforcement
Actually they can. Just because a device has passed test does not mean that it is complaint with the EMC regulations when actually deployed.
A device can be fully complaint with the relevant EMC standards ( & these Powerline device are fare from compliant) yet not comply with the EMC directive when operated. one of the essential requirements of the directive is
"the electromagnetic disturbance generated does not exceed the level above which radio and telecommunications equipment or other equipment cannot operate as intended"
Enforcement where devices are preventing radio services operating as intended is the responsibility of national regulators, in the UK's case, OFCOM
EMC directive is readable in full here.