As observers have noted, this time around, Obama slightly shifted the previous red line, removing any reference to "moving around" CW, as Assad had already crossed that line with no consequence. The red line now is only about actually using the weapons.
There are plausible scenarios in which Assad would use CW in a tactical manner against his domestic enemies-and it's not at all clear that he wouldn't get away with it. Assad will fight tooth and nail to maintain control over Damascus, while also securing the route from Homs to the coast (an area that witnessed regime ethnic cleansing attacks). As I noted in July, the CW are Assad's insurance policy to protect his retreat into the coastal redoubt.
At the time, some in the administration had a similar reading, placing the potential use of CW in the framework of a "targeted ethnic cleansing campaign" by Assad, and proposed that he could use "the threat of a chemical attack [to] drive Sunnis ... from their homes." Seen this way, CW could work just as well to maintain a grip on Damascus by forcing hostiles out and keeping them out.
It's true that the administration has warned Assad against using CW against his people, but it's doubtful that Assad finds Obama's threat credible. For one, the administration has loudly made it known that securing the CW sites would require 75,000 troops-effectively ruling it out as an option. Besides, Assad has seen Washington ignore other benchmarks -- such as the use of fixed winged aircraft, cluster and incendiary bombs, and now apparently Scuds or "Scud-type" missiles -- and has probably concluded that Obama is unlikely to send in the cavalry should a few more hundred Syrians perish in a tactical chemical attack.
What's more, Obama has now offered Assad another loophole with the designation of the Jabhat al-Nusra group as a terrorist organization. As soon as news came out that the designation was forthcoming, the regime rushed to claim that rebels had seized control of a toxic chlorine factory in east Aleppo, and may now use these chemicals in an attack. Such bogus stories set the stage for a possible attack in the future and provide Assad, and his backers in Moscow, with enough to muddy the waters.
Similarly, there have been opposition claims that Assad has already used CW. It's difficult to verify these claims, but that's all Assad needs. Recall how at the time of the Houla massacre, many paused and wondered if this wasn't an attempt by the opposition to force an intervention. Others claimed it was the opposition's own doing. There were no good guys in Syria, after all, just like there were no "good options."
The question Assad likely asks himself is: Would the U.S. really intervene over a deniable incident, the facts of which may not be clear, and that might claim the lives of a couple hundred Syrians when it has sat idly by as 40,000 were killed?
Assad is banking that the basic parameters of U.S. policy will remain the same. The administration's performance this past week, going back to July, probably reinforced his conviction that not only are his CW a useful bargaining asset, but also that the odds are decent he could get away with it if he used them shrewdly.