I think it's pretty telling that comments surrounding an article about artists not complaining about getting paid enough people complain about the only method Spotify can implement other than charging you money for the service.
It's not a rude behavior and is within their right when you utilize their service for free.
Commercials/advertisements are fine.
Allowing over compressed, perceivably louder sounding commercials on their system is annoying.
Recognizing that people hate this and trying to prevent people from turning down the volume in reaction to a jarring change in sound is rude.
Being rude to your free users in order to get them to upgrade is a strategy that works (nagware), but is it a better than other strategies?
It's not a rude behavior and is within their right when you utilize their service for free.