I’ve given this a lot of thought.
Santa, horrified by the abominations that are the Misfit Toys (perhaps created as part of a secret North Pole research program that went awry), has banished them to the I of MT where he was confident that no one would ever learn of their existence.
Little did he know at the time that Rudolph and Hermey, shunned by the mob mentality of the elf & reindeer communities would stumble onto them and promise that Santa would come and save them.
On Xmas eve, flying through the fog, Santa had no idea until they landed that Rudolph was taking a detour to the I of MT to save his mutant friends, so he had no choice but to smile grimly and accept the new payload or risk the exposure of his plot.
Note that we never see where the Misfits end up — they’re each handed an umbrella (wait — each? Look again — the bird who sang about not being able to fly does not receive an umbrella!) and pushed out of the sleigh into the fog, not knowing until far too late that they’re the first ones out because the sleigh is still over the Sea of Japan.
They spun in.
There were no survivors.
HO HO HO