It's still just a matter of poor policing and over-stretched police forces. Many people, police included, wouldn't know at a glance the difference between a legal e-assist MTB and a 4000w e-MTB. I'd have thought it would be an obvious stop if someone was seen doing 30mph up a hill, but you never hear of anyone getting done for it unless they've killed a pedestrian.
Same reason why there are loads of 2 stroke motocross bikes with L plates knocking about; the police assume that becasue they are 125s then they are learner legal. I saw a KTM 125 SX with L plates the other day, which makes 38bhp (the limit is 14bhp for learners). In fact I have seen this bike on my commute for quite a few weeks so they presumably haven't been pulled yet.