Why would he do that?
When the President sign the bill into law he stated (source):
"I also have reservations about the constitutionality of the broad ban on issue advertising, which restrains the speech of a wide variety of groups on issues of public import in the months closest to an election. I expect that the courts will resolve these legitimate legal questions as appropriate under the law."
It appears to be his last opportunity to stand up for his statement.
See I was wrong already...
Section. 2.
Clause 1:
The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Offices, and he shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.