IHS vs IH Sigma

In a post that discussed Obama's request to cover up Christian imagery at Catholic Georgetown University, the etymology of "IHS" was also discussed:

the most common Christogram is "IHS" or "IHC", derived from the first three letters of the Greek name of Jesus, iota-eta-simga

Is this mixing of two different alphabets an early example of PC multi-culturalism? Or couldn't the Romans find a sigma symbol on their keyboards? /sarc