Source discrimination is bad. The Internet allows applications and services to run “at the edge” of the network and not centrally; this encourages innovation [1].
Path discrimination is more complicated. Many content providers already pay private networks to transport their traffic on faster than the public internet [2]. There is even a market for traders paying tens to hundreds of thousands of dollars for low latency connections [3].
Given that ISPs charge each other for transporting traffic [4] it costs more to broker traffic across others' networks versus its own. It doesn't seem unfair for the ISP to charge less for the latter. This would allow Comcast et al to compete with the Akamais and Level3s that irk them today [5]. Sooner or later they will find it makes more sense to offer the discount for same-network traffic to everyone.
Comcast has made shitty statements about net neutrality before [1]. But it's not okay to vilify anything Comcast says by virtue of it being said by Comcast - that's straight up ad hominem.
[1] http://www.stern.nyu.edu/networks/Economides_Net_Neutrality....
[2] http://techcrunch.com/2010/11/11/level-3-lands-netflix-strea...
[3] http://www.highfrequencytraders.com/article/682/options-it-o...
[4] http://blog.teracomtraining.com/how-isps-connect-to-the-inte...
[5] http://blog.comcast.com/2010/11/comcast-comments-on-level-3....