I don't know and I see your point. Language is powerful though. And if the word porn is used by all and sundry all over the shop it kind of makes actual porn more accessible somehow, not as seedy, not something furtive but out in the open.
Interestingly I don't agree that "porn describes something in a way that diminishes its value". Porn in any context to me primarily means something to excite the senses of the viewer.
So property porn is where the property is shot and described so lovingly that watching it is a sensual experience...