A modern fire-resisting door leaf doesn't need to cost much more than a non-fire resisting door leaf; the frames (for FD30 at least) need not typically be better timber and are often pre-machined on one face for the intumescent strips (which need only cost a few quid per doorset). So, what sometimes happens is rather than running the risk that they won't install a fire door where it's wanted, the chippies will install fire doors, including intumescent seals, everywhere, 'cos it's easier, less risky & doesn't cost them very much.
BTW that fire resisting doorsets are more robust isn't necessarily true - there are some perfectly adequate fire doors that are made of little else but chipboard with a hardwood lipping on each long edge.