The reason that there is a limitation on the correction required is that with high myopes there is a higher risk of retinal detachment.
It is indeed possible to do the route FAA Class 1 + CPL/ATPL and then go for a JAR Class 1 and be OK if you meet the renewal requirements which are a few diopters more.
Why are they 'more lax' for renewal than the 'initial'?
The logic is that young people's refraction error can get greater, so for someone to be be just acceptable; then shell out £60k+ only to fail the renewal standard would be particularly hard.
The FAA just has one standard.
Different cultures different philosophies; you can argue what is just either way.
The above is how things stand at present and are no guarantee for what happens in the future or on your application.
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