I seriously doubt it is an oversight with the exhuast I just wish we knew the reasoning behind it. 
Pushing limits as they know the other teams are keen for unity and will possibley not question it. Same as the winglet on the ferrari in front of the sidepods. That goes against the concensus agreed by the teams to get rid of apendiges but its not been questioned. Wonder what else they've slipped through.
Pushing the limits is what Formula One is about. Innovation does not come from stopping when it looks like you've gone as far as you can. Every team should be trying to rest right on the lines of legality. If they're not, they aren't competitive enough. There are a lot of things that, to me, are questionable but if there wasn't a high chance that they were legal, then they would not be on the cars.
As long as they are legal and conforming when the season officially starts who cares?
Quite a few people, it would seem from this thread.
I dont care what they do before the season starts but the worry is that they will continue when the season starts. Look at the rear view of the ferrari that tex posted. Those mirrors are postd on vertical vanes , the sort of thing that was not supposed to be on the cars this season, the sort of thing that causes air turbulence in the cars vwake. And there they are on the ferrari bold as brass, "a feature that clearly gets around a commonly agreed intent" The quotes are from a Mark Hughes article in this weeks autosport. So what do the teams do? complain that this is not what was agreed or let it go as harmony amongst the teams is of greater importance. Geez why cant Ferrari play by the rules once in a while!! 

Mark Hughes started his article with a quote from an old timer on seeing the Ferrari "What the hell's that? I thought all that crap was supposed to be consigned to history with the new regs."
You're using this as an anti-Ferrari dig, not a dig at potential rule- breaking. Otherwise, you'd be picking up on all the other examples of things that we thought we wouldn't be seeing anymore. Extra bits on Renault's end-plates look like flick-ups that should be banned. Similarly, there are shark-fins, and Toyota have something similar to Ferrari's mirror-winglets, but they are far more obvious.
Even McLaren are not exempt from this (and you really should've looked before you started ranting). There appear to be little aero parts on their wheel-covers and I'm sure that those shouldn't be legal as they could disturb the airflow, just like anything else you might glue on to a car.
Pushing the limits of the rules is perfectly fine, as long as the finished product doesn't over-step the line. Since none of the cars have been declared as final, there really is no issue here.