its not illegal....in my point of view...
they're smart to tweak the construction of the crash structure, and maintain the flat diffusor that will meet the regulations that was "written" by the FIA and the "over-taking working group"....
Brawn, williams and toyota were not in the overtaking working group and that was how they interpret the rules and smart innovation to increase downforce and better aerodynamics....
about overtaking working group....
http://www.autosport.com/news/report.php/id/61511