I suspect that that is exactly what is behind this policy. Amazon and especially the major publishers do not like the prospect of a two-tier market with DRM-free “full” eBooks and protected lesser ones.
The music industry had the problem that there was a much more established tradition and expectation of DRM-free music, both in the form of legal CDs and often illegal MP3s that were mature and ubiquitous long before online stores became relevant.
