Remember it refers to followers of the Dhamma. The motive of gain is there and they get what is necessary for survival by almsgiving. So for them it counts as greed. If they can heal (not applying occult practices or techniques, of course), very well and good,but expecting nothing in return, This is how I see it.
The case of healing as regards doctors of medicine is different, even though THEY practice primarirly to make a decent living for themselves.So the merit is mostly on the mundane level ( for they get their reward in the here and now) and so it decreases on higher levels the more they charge. Compare: there have been a few ‘doctors of the poor’ here where I live. They do not charge money at all.
y not