The country of Macedonia was part of a region generally called Macedonia for over a thousand years, as the initial named unit of Macedonia (which was in Greece) expanded to cover a wider area.
Besides I think the people in Greece-Macedonia are happy being Greek.
Macedonia is part of the Greek identity. FYROM (still the official name until the agreement gets implemented in a few months) have been claiming this Greek identity by misappropriating history.
Think of it as a trademark violation.
I wonder - is this spelling some sort of shibboleth for Greeks who want to make a point that 'Macedonia' is only the area in Greece? I've seen it a few times in this thread, but never before anywhere else (then again, I didn't even know this was an issue until a week ago).