This, I believe. No idea what the cost of registering with hmrc is but surely its not more than the potential loss in sales?
And even though the £135 figure is set by HMRC and not the retailer, I still think its arbitrary.
Clearly I don't know the full machinations of international goods and services, but I don't have to; if a retailer doesn't want the expense/bother of being able to sell below a certain price point, they lose out. People will spend their money elsewhere.