The search space is all tunes that have been played on Soundwave from all sources plus a large source with a huge catalog. Soundwave sources include Android Native, iTunes, Spotify, Deezer, Rdio, Pandora, Deezer and a few others. The authority source has a about 20 million tracks.
Bandcamp is not something we have support for right now, but it's been requested a number of times, and we like to listen to our users. Soundcloud support was a result of a very vocal group of users with the same perspective as you have.
I had this same issue. Apple wouldn't help me, unfortunately. If I recall correctly, I had to disassociate iMessage from my AppleID completely, which seems to have worked. Although I still can't be sure.
I have a weird feeling the messages still aren't going through and a lot of my friends think I'm being an asshole.
This feels exactly like the dark days of Microsoft[1], where they would support a protocol halfway, and then make it impossible for people to switch away from their version of the protocol.
Honestly, this is great for Twitter. I can't see them blocking this. I can either advertise my product on Facebook which necessitates leaving the site in order to buy my product, or I can use Twitter Ads with #AmazonCart and see usage on a case-by-case basis all while staying within the confines of Twitter's app.
