You answered your own question – they're VERY expensive and most people in bike shops just want to finger things, try them on and then wander off and buy them online.
It's preventing people from doing that with an item that the shop has spent a fortune on. How many shorts in Assos range? Get in a range of sizes and models, average price of £100 per pair and you're looking at an outlay of thousands of pounds even at trade. Punter comes in off the street, greasy hands, feels them all, tries a few on, rips one pair but just puts it back quietly then decides he's going to wander off to [insert name of online retailer].
Well the shop has just got wise to that! If people want them, they'll stump up the cash. If the item is unsuitable, the shop will refund or put the money towards another item.
Electronic tags aren't a deterrent to serious thieves, especially not if you know how they work – they're a doddle to remove if you know how (thankfully most thieves don't).