But again this is really the point; calling for peace alone is hardly treason. Or rather, treason itself is a fairly meaningless offence, because it essentially amounts to "Did you work against the government, and did you lose" rather than a genuine indictment of working against one's country.
Consider a few of the following:
Guy Fawkes
Benedict Arnold (twice)
The Easter Rebels
The French Revolutionaries
Klaus Fuchs
I would argue that of these, only Fuchs was really a traitor (okay, maybe Arnold the second time); he was motivated to work against 'his' country and government. Guy Fawkes, Benedict Arnold, and the French Revolutionaries were all working against the state, but not necessarily against the nation; in their minds they were working with the national interest in mind to achieve the best for their country. Whilst all were certainly guilty of treason they were not moral traitors so much as people with a different set of political values. The Easter Rebels meanwhile were also clearly traitors, and also clearly working against both their government, and their country (the United Kingdom of Great Britain and Ireland), but did so in support of what they saw as actually being their country (Ireland in its own right).
Petain falls into the same category as Fawkes, Arnold, the French Revolutionaries, etc... IMO.