Bryan, The Retweet that you mentioned would not carry any RT/MT
indicators, nor can it be adjusted, as the original poster had
mentioned.. He is referring to the old fashioned retweet (i.e. before
the retweet button came. Some people,including me, continue to use this,
 when they have to add a comment or even a LOL), where you clicked reply
 and added RT manually..
The norm is that in case you left the original tweet intact and any
comment you added are outside of it, you should be using RT. If you
modify the original tweet (maybe to adjust length, or dropping an
irrelevant part), you should be using MT (Modified Tweet), and not RT,
as any Grammar mistake or factual distortion may be attributed to
original tweeter, if RT is used.

The answer, of course, is that many people still don't know what MT is, and i guess the solution is simply to use this more often, make people google for what it stands for, and gradually, this would become as universal as RT.

See question on Quora