Perhaps Pam didn't want to put people on the spot by asking them which cake they wanted, which would force them either to snub one or other of the cake bakers or to look greedy by saying 'Both, please!', and thought best to give everyone a slice of both and leave them to eat whatever they chose. (Another solution could have been to cut slices of each on to a platter and hand it round to everyone; but not everybody's table is suited to that.)
If you were in time to forestall her planking both kinds on your plate, you could without offence have said, perfectly truthfully, 'No sponge cake for me please - it's just not my thing'. If not, you could simply have left it. No sane host/ess expects every guest to love every item they serve, or requires them to eat everything whether they like it or not; and it is rude to pay close attention to, let alone comment on, what guests leave on their plate.