I experienced the same problem. The customer was not informed and not given an option to opt out. I complained and later the CS came back, my actual usage charge while I was in SG was only half of what was under the Maxis One World charges.

When I insist on opting out, the CS asked me to manually select other telcos other than Singtel. But, that cannot be done as it was auto defaulted to Singtel. And when I complained again, suddenly my roaming telco was changed to M1. So seems, customer can opt out. The question is whether Maxis wants to give this option or not!!