That they were convicted is without question just - but the sentence they received may have been to hard, I'm not informed enough to judge that.
Here's their major offence to Russian society, proving they're a danger
http://www.telegraph.co.uk/news/worldnews/europe/russia/9482190/The-punk-prayer-that-landed-Pussy-Riot-in-court.html
the "song"'s last section was an obscene parody of the Christian liturgical hymn the Sanctus, substituting the word “shit” for the word “holy”.
the “punk prayer”, accompanied to riotous music and dance, was filled with expletives and profanities and used grossly scatological language.
you are not going to change your mind do you
that's it I'm done with this
The USA believers cannot read
If that's the argument, here's the major issue: the majority of people in the West, not just the US, are still wondering how this cannot be considered a disproportionate response. Seriously, if you want desecration of religious icons, you don't really need to go much further than Piss Christ, where an artist literally submerged a crucifix in a bottle of his own urine in the name of art. Yes, it drew public ire, from both religious and public officials. No, however, it did not get the artist arrested. Australia has not prosecuted anyone for blasphemy since 1919. The UK banned a film for blasphemy in 1989 (the same year as Piss Christ, actually), but didn't arrest the people responsible, much less sentence them to two years; the last such imprisonment was in 1921. In the United States, since you yourself bring it up? 1838. Substituting "shit" for "holy" being worth two years of prison time? Even a majority of
Russians familiar with the case (sorry, Russian link) apparently thought it was either excessive or that it should not have been tried in criminal courts in the first place.
EDIT: Clarified the particular Russians - the relevant question was only asked of those Russians who stated they had some familiarity with the case.