Because that is what our president said:
"We have been very clear to the Assad regime, but also to other players on the ground, that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized," Obama said. "That would change my calculus."
I can understand a strong response if Assad starts using chemical weapons against civilians and insurgents.
The movement angle is interesting, depending on what Assad wants to hold, since the chemical depots are reportedly in four areas:
The U.S.-based Global Security website says there are four suspected chemical weapons sites in Syria: north of Damascus, near Homs, in Hama and near the Mediterranean port of Latakia.
The article says the president mentioned movement in a threatening manner, but the quote used doesn't say that and there is no clarifying quote. So it seems like just movement is considered a threat.
If Assad tries to hold all of Syria, obviously he doesn't need to move any chemical weapons and their launchers.
If Assad tries to hold the Core Syria, stretching from Idlib and down an arc through Hama, Homs, Damascus, and down to the Israeli and Jordanian border, Assad also doesn't need to move chemical assets.
If Assad retreats to an Alawite homeland plus a buffer running from Aleppo to Homs, he needs to move the Damascus assets.
And if Assad retreats to a mountain Alawite homeland only, he needs to move the Damascus, Homs, and Hama stockpiles.
Or if not move, destroy what he doesn't take to keep them from getting in the hands of al Qaeda types who'd use it on Assad's forces. Or did our president just tell Assad to abandon any chemical weapons outside whatever perimeter Assad decides to hold? If so, why would we do that?
Or would Assad just abandon the chemical weapons just to spite the West and figure securing chemical weapons will keep the West too busy to interfere with a retreat to a realm more easily held?
I have to believe we'd need airborne troops to drop right on top of the sites with relief columns racing in overland to link up. I can't imagine that air strikes are an option under these circumstances. What level of bombing would be required to burn up chemicals rather than crack the bunkers to let gas escape to affect nearby civilians or allow looters in?
Unless this is just bluster on our part, the implications are interesting, to say the least.