Yes, it's in thire fare rules or CoC, have seen it before...
its a ridiculous fee ($100) that was deemed invalid somewhere in Europe I think because there is no loss or cost to the airline...
If you are not a Mabuhay Miles member and are not planning to fly with them again (soon), I'd consider not paying - how will they enforce it?
I don't get how this is an issue. Most things airlines charge for have no cost or loss. And this one seems like it does have a cost, as you checked in, and have a seat on the plane?