Just read the opinion in Perez v. Mortgage Bankers Association. Just want to clarify the holding was addressing interpretations of rules, not the creation or change of the rules themselves.
http://www.supremecourt.gov/opinions/14pdf/13-1041_0861.pdf
It's not a major difference, but an important detail on some issues. With BATFE, considering interpretations of rules on things like the SIG arm brace, the ability to change interpretations on a whim is significant, especially with the popularity of the arm brace and the number of pistols now featuring that device.
It does get them out of the need for the comment periods etc. but it doesn't get them out of the court challenges. If anything, it brings them on quicker.
And the ATF already has quite a few court challenges lining up against them, the '86 Machine Gun freeze/Hughes Amendment, the 41p rule on NFA trusts and CLEO sign-off, and now the Sig Brace.
Adding a fourth with the M855 ban, one they were on extremely shaky ground legally on, some might say quicksand... it just wasn't wise.