So basically, you're saying that since they (according to you) weren't plastering news about their P2P design across all their marketing (as if most people would even be able to understand that, from a marketing perspective), and DESPITE the fact that it was well-known enough for academic papers in 2011 to be written on the design (http://pansentient.com/2011/04/spotify-technology-some-stats...), that's why you'll never return to them, even though they've since entirely removed that functionality? Because you never heard of that part of the design until you detected it on your own network?
This isn't someone "robbing your bandwidth" IMHO, I'm not for example angry when I torrent something open-source and have it set to reupload 2x what I downloaded... In fact here's Netflix debating going to a P2P design in 2014: https://arstechnica.com/information-technology/2014/04/netfl...
If I torrented for a year ("as a downloader") without realizing how it worked, and then found out it was uploading what I already downloaded, would I have the right to be indignant about my own ignorance?
That said, a lot of new endeavours (especially on the Internet) have questionable legality when they first emerge (mp3 scene, bitcoin, digital content distribution in general, etc) and then acquire legitimacy later on. It's fine (... I guess?) to feel unlimited righteous indignation about a company (I feel that way about Microsoft, actually), but it's not fine IMHO if it's not based on reality.