I agree, it was a real kick in the teeth for the UK when John Kerry described France as the USA's oldest ally
Little more than a diplomatic attempt at a slapped wrist. If any 'western' states were going to get involved other than the UK, it was always going to be the US and France.
Obama has little choice having backed himself into a corner by publically showing his hand as to what does and doesn't constitute a 'red line'. Having drawn the line early he now has to do something if he is to maintain credibility and not appear as a paper tiger for the rest of his term. But paradoxically, that he can't be re-elected means that he also has the flexibility to make such mistakes.
Hollande, well, other than desperately needing a distraction from the rapidly growing pile of socialist disasters he is inflicting on his countrymen (even his ministers have come out and said they have over done it on the tax front), Syria was always within the French sphere of influence. In the WW1 era, the Levant & Middle East was "neatly" carved up between Britain, France and Russia under the Sykes-Picot agreement with lines drawn on maps that put present day Syria firmly in the French sphere of influence. The French are not getting involved out of any sudden desire to appear punchy or to get in bed with the US - there will always be a degree of distrust between them and I very much doubt this will see a sudden UK-US divorce; the French are getting involved because it suits them to do so. In foreign policy and international relations terms, an archetypal realist response.