Quite simply because they've got away with for enough years to create a situation that is thought to be beyond correction.
Essentially the strict 200 watt limit that originally existed in the UK made e-bikes virtually useless, the added power only cancelling the added weight. So after over a decade of that manufacturers started to creep up the power, that first noticeable from about 1999. It's no coincidence that e-bikes started to become more popular in the UK from then on.
Confusion increased when the EU introduced the 2002/EC/24 type approval law for two wheeled motor vehicles that had some exclusions, one being that pedelecs having 250 watts watts maximum assist power with a maximum assist speed of 25 kph and weight under 40 kilos were exempt from motor vehicle type approval. That was mandatory of all member states with a deadline for introduction, and that was written into UK law on 10th November 2003. Accompanying that was an instruction that member states had to remove all conflicting national legislation, and that was where the UK fell down, that was not done.
Of course that wasn't a law making 250 watt e-bikes legal in the UK, though it looked like it, so from then manufacturers and agents increasingly openly promoted their e-bikes as being 250 watts, mistakenly or maybe conveniently thinking it acceptable.
However, the DfT had long recognised that with the EU having control of transport policy thoughout the union, parity of pedelec law was inevitable, and so they regularly promised it "within a couple of years or so". As ever, such minor matters always get lost under the pressure of what governments consider more urgent matters, so it never happened. The current promise is that the revision of the legislation will come into force in 2016, but since that's obviously just a year of convenience being after the next election, take it with a pinch of salt.
With e-bikes gradually becoming more common in some parts, it was inevitable that sooner or later they would come into conflict with the law, and that happened in one particular case when a traffic officer doggedly pursued that matter of the 250 watt rated power when the law only allowed 200 watts. BEBA got involved with me behind the scenes helping on the variety of all aspects of UK e-bike laws (there are many more than I've mentioned here), and the police force involved found themselves being apparently blocked from action by some of those measures. That force therefore took the matter to the DfT who, faced with the opposing arguments, finally realised that something had to be done.
That something was a meeting at which the representations from a consultation of interested parties were discussed and in recognition of the large number of 250 watt rated e-bikes on the road, the DfT agreed a waiver for them, here's the relevant extracts:
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Summary of Government response to consultation
2.The Department for Transport has considered the responses to this consultation and supports recommendations to harmonise power limits (from 200 Watts to 250 Watts) with similar provisions in place across Europe. This will provide consumers with access to a wider range of electrically assisted cycles.
Extracts from intentions:
The DfT and Police are aware that electric bikes currently sold and used in the UK are fitted with 250W motors, they do not intend to become involved in prosecuting those that sell or ride them. The use of an "Off Road Button" however is strictly forbidden now and is specifically mentioned in documents appertaining to new and existing guidelines.
"Grandfather rights" will apply to all bikes sold before the new regulations come into force. So any changes will not be retrospective.
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As you can see, that does not give an open door to any power as some say through their readings of EN15194, since that has no force in the UK at present. The only legal method of power rating for e-bikes in the UK at present is specified in BS1727 from 1987.
Note, no laws have been changed yet, we are still legally bound by all the 1980s UK e-bike laws, so liberal interpretations are dangerous.
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