It's also worth pointing out that there are several levels of people who will prevent you from opening your checked bags (if they can catch you) in the customs area and before you put them back on the belt. The customs agents will be over to you in a flash if they see you tugging on zippers before you clear customs, and the men who man the belts will attempt to keep you from opening them after customs if they bother enforcing the rule which is posted all over the place.
It may be the they just depended on these people before the security was put in place.

Another hazard: due to construction in the last year, the N-S subway hasn't been running. This means that what was previously a very simple connection from arriving SEA-NRT UA flight in S to a departing UA flight in N now entails going through security into the N gates. If you're not PremEx or higher, that can mean an hour or two (at that hour of the morning).