I'm not agree, at all. And, please, allow me to explain why [SPOILERS]:
1.- The magnitude of the crime itself. It maybe not very relevant, but it is. Even if Loghain and Rainier did what they did for greed and blind ambition, killing some orlesian noble (that, in fact, is going to die anyway, because if a general ask for it, and I don't accept, any other Captain will take for me) it's not the same than betray a king, and just let hundreds of soldiers to their death, or worse (not everyone die, some can become Darkspawns or worse, can end like prisioners to suffer a long and very painfull death) and put an entire kingdom at risk, almost destroying grey wardens in the process... You get the point.
2.- Something that I really hate. Loghain's plan was sloppy and full of holes. It close more to a civil war than a real "coup d'etate". At least, Rainier gets the job done, at does it nicely.
3.- Loghain never repent for his crimes. What he regreats it's being defeated, and accepts it's with grace, but that's not repentance. He deserves to die, and more important, he deserves to die as a traitor, and it's not important what he done in the past. Rainier, in the other way, will take that charge all his life. And that's a far worse sentence than anything you can do.
4.- Rainier's men are not innocent. The "I was just obeying orders" has been never a valid excuse for me, except in very rare cases. Rainier may be the one to order the attack, but the swords who killed the woman and children of the orlesian noble are not his. This don't make Rainier innocent, of course, but makes the men guilty. So they deserve what they got.
4.- Don't forget, that from Inquisitor perspective (that's a role play game, after all) Wardens have failed to everything they say to live for. They have even provoque a blight, or something even worse. For the Inquisitor, the grey wardens are, in the best case scenario, incompetents. In the worst, a bunch of false, blind and coward traitors. Actually, just Hawke's friend and Blackwall are the only "wardens" that fight against true evil and make the right thing. So, from now on, The wardens are not the saviours they claim to be. Just another corrupted and guilty order, like the templars, or the chantry and the circles of mages. They don't deserve to judge Rainier when they have their owns crimes to respond. And they have to win they respect again, in the eyes of the inquisition.
5.- Rainier never really do anything against Grey Wardens. The darkspawn killed the true Blackwall, and Rainier just makes what he thinks Blackwall'd have done. And he never passed the joining, so he really is not a warden. It's a pure Inquisition inner problem, and a problem for Orlais authorities, at most. But the Wardens here do have nothing to do.
6.- He doesn't go "free" in any case. He serves in the Inquisition. That's not a hard punishment, maybe, but it's not going free, either. From now on, he has to do everything he does, and do it even better. He received a second chance, but the eyes of Inquisition now are fixed on his neck.