Potentially because you're no longer in the country the SIM came from, but that plan is still the best deal for you, e.g. you got a Three SIM in the UK at the beginning of your six-week trip but on day 31 you're now sitting in France where the prepaid options are pretty terrible. It's a lot easier to throw 15 quid on your Three account than go hunt down the least bad French prepaid SIM.

On a related note, do we have a list/wiki anywhere of which carriers take foreign cards directly for top-ups and which don't?