That's not strictly the case. Decoder cards are legal but FA permission is still needed for pubs to show foreign coverage. Monopoly rules would prevent the FA doing any blanket bans but it would still be entitled to set competitive conditions.
It's fantastic news for individuals though because private homes don't need any such permission and may simply get installing the cards...
Not sure the copyright clause will necessarily hold up on this, if the Pub bought a subsciption for a bar from the Greek company the permission is inherent in the contract between them and the tv picture provider for all the elements to be aired to the public. The only way I can see the pub not being able to continue to show the matches is if the Greek decoder is piggy backing the British broadcast signal from Sky rather than delivering the programme as it's broadcast in Greece.
On the question of individual use of signals from other countries it already happens and is almost impossible to stop, a friend of mine has a villa in Menorca and simply takes his Sky card with him and inserts it into a second decoder box connected to a dish at his villa, voila all the British Sky tv as usual. Other full time ex pats in Menorca simply use any friends address in Britain as the billing address and then use the Sky card year round in Spain.