The crew did not contravene the regulations of the FAA or the CAA! Can you read English? They did exactly as 4 engine long haul crews, American, British, Australian and worldwide have been doing for years
Actually
Rainboe, what the FAA for years did allow is no longer the case. The interpretation, not the rule, has changed. On five occasions I've shut down one out of four and happily, paying due deference to proper contingency enroute planning, continued to destination.
Six years ago when I last played the game nobody wearing a suit with the matching management mask accessory said 'thank you'; instead a finger of admonishment was waggled in front of my face. I was told not to do it again and the reasoning given actually made sense having to do with compliance with the spirit of the regs rather than safety - a new FAA interpretation for a brave new world of improved safety. It was made perfectly clear that I would be placing my certificate in jeopardy with possible action to be taken against me should subsequent events additionally go awry (such as landing at MAN rather than LHR due to a flap over the FOB) if I was to press on after shutting down an engine.
The case that the feds are going to bring is one based upon prudence and judgment rather than safety. For what it's worth, in my book, 3 out of 4 always beats 2 out of 2, but my book counts for little as I'm not the one interpreting and enforcing the rules. Nothing theoretical about this, operating in US airspace (under the FARs) with one out of four shut down no longer means carry on regardless.