For that matter...why do EA/Bioware think the game is worth $60 when every other game in the franchise was the regular $50?
The console versions were always $60 and EA has started charging $60 for all their high-profile PC games as well (Crysis 2 and Dragon Age 2 were also $60 on PC). Same with Activision and Bethesda. It's not unique to ME3.
Are they justified in charging $60 for PC ports? No, not really. A PC port typically costs less than $1 million to develop and publishers don't have to pay any royalties for PC games. In addition, if a game is sold directly through Origin, EA makes 100% profit. They're trying to raise the standard MSRP so they can make more money, not because the games are actually worth that much.