Well, the reason Cyric can't interfere is because he (and all other gods) are, indeed, forbidden to do so by Ao.
They are forbidden in the default game, yeah. But I think that it doesn't really make much sense, if you look at the history of Faerun; why would CHARNAME get a special treatment from Ao? I think that the Throne of Bhaals developers simply wanted an excuse as to avoid the added complexity of divine politics.