It wasn't an Ed Sheeran song he was singing. It was a song from one of the later books. The song was about Tyrion and Shae's relationship... Songs about the current royalty of the world being a common thing in Westeros (i.e. the more famous "Rains of Castamere"). The "it's a new one" wasn't a wink to the screen breaking the fourth wall. The song was literally new to the world of the show.
Snow Patrol was also on the show. They're arguably a bigger band than Ed Sheeran. Didn't see a single person jump on the complaining bandwagon then.
I am aware that the song is from the book (admittedly only because it has been mentioned in the various articles/debates I've read these past few days), but the point is that it is an Ed Sheeran song in that it is completely typical of him and is being sung by him. He could do a rendition of pretty much any song (especially something folky such as this), as he has done before, and it would still be an Ed Sheeran song. I'd say that the "it's a new one" was clearly implied as a cheesy wink & nudge... and even if it wasn't, you'd expect the producers to have considered whether it would be taken as such by the public. If not, then it was somewhat of an oversight given the evident backlash.
And I guess the reason that most people didn't complain about Snow Patrol is because they didn't recognise the guy from Snow Patrol, and if they do, they didn't see him as a pop brand, which is the whole point. Perhaps it's more of a UK thing but Ed Sheeran is
frequently plastered all over magazine/newspaper covers/tv/radio etc, ergo it
is different. He is clearly a brand... Gary Lightbody from snow patrol, not so much. Brands have have their time; I'm sure if Ed Sheeran did a cameo in thirty years well after he was at his peak, then it would be more of a "who's he again? Oh, him." type thing. As it stands, that is not the case.
To me this is just pandering to the mainstream, to the detriment of the immersion. Again, I'm not massively bothered, although I do hope he doesn't play any kind of remotely important part in upcoming episodes.