Possible reasoning, if indeed Assad did initiate it:
The US doesn't want to intervene, but (unwisely IMO) committed itself to action if a "red line" is violated". Always worth making the other guy do what he doesn't want to do.
A US strike will be limited: essentially the purpose of the strike is not to have an impact on Syria, but to show that the administration backed up its "red line". The strike will thus be survivable.
A limited strike will gain the Assad regime the Muslim world street cred of being the ones fighting the Bad Americans without subjecting them to excessive risk. It also puts his antagonists in the uncomfortable position of being allied to the Bad Americans.
A US strike would give the Iranians an excuse to intervene more openly.
Not saying that's what happened (like the rest of us, I don't know what happened), only that there could be some internal logic to an Assad-sponsored chemical strike.
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