I think the easy way of telling who's behind it is if anyone else offers the same thing. If Play (and [insert online retailer of your choice]) also send out full replacements unprompted it's probably a CBS (or whoever distributes them in the UK on their behalf) decision, if it winds up being just Amazon it's likely something they've instigated themselves.
People who've brought from shops are going to be more buggered as they'll have to take them physically back and either get a refund or hope whatever else they have in stock is the fixed discs.
If I can walk into Best Buy and exchange my set, I'll be perfectly happy with that. Just don't know if that will be possible.