You'd need to ask the woodland trust that question but I get the feeling it's because it is rare thing to see wild in the UK. It could also to be something to

You'd need to ask the woodland trust that question but I get the feeling it's because it is rare thing to see wild in the UK. It could also to be something to do with the Plant Health Act 1967 which is aimed at stopping the spread of <insert thing you're trying to stop>. Inadvertently Introducing new fungal varieties into a woodland may cause damage to the woodland. I don't know for certain though but probably down to a different take on policy approaches to ecology between our two countries.