Why do they require connecting passengers to re-clear security? That is just nuts

I've done the gate check dance with the TSA for years, so I know how to scan the situation and avoid them - the Japanese version shouldn't be any different.

What might concern me is connection security making a fuss over carry-on size - how strict are they at these checkpoints? I don't want to get into an argument about a 22" carry-on that went just fine out of the US and out of Taiwan, but all of a sudden is too big for my connecting flight to Taiwan or back to the US.