Simple answer: there is no exception within the 2nd authorizing the prohibition of arms by felons. The only way to change that is by amending the Constitution. Thus, by a plain reading, there exist no Constitutional authority to prohibit the felon class from keeping and bearing arms - no matter the emotional appeal of it.
If a criminal is determined to still be a danger to society they should not be released back out into society.
Prior to that time, you could go away for 10 years for murder (or whatever), get out, plunk down $79.95 and walk out of Western Auto or Sears with an AK47