The French are not only 'pissed off' for commercial reasons. The deal was part of a long term strategy of France in the region, with India and Australia being the two main partners, so this has serious consequences for them beyond a simple arms sale.
Now, on the other hand, two good questions are whether the Australian ever discussed the possibility of France supplying nuclear-powered submarines instead of conventional ones, and whether France would have accepted. I don't know the answer to either though I'm doubtful about the latter and that may be an objective and very good reason for Australia to switch.
And yet it's a tacit agreement among nuclear nations to not sell military nuclear equipment to non-nuclear nations (ie who don't have at least a civilian program).
Which is why North Korea for ex has never been able to buy nuclear equipment from China or Russia. So far, because now that the US has decided breaking the rule for a short term financial gain was worth it, other's will decide that they might as well help their own allies too with a few reactors.
There is not nor has there ever been any prohibition on providing nuclear technology and equipment to non-nuclear weapons states. In this particular case, France offered to sell Australia nuclear submarines back in 2016 and Australia decided against it, but one of their reasons for going with the French proposal was that the submarines could be converted to nuclear in the future.
In 2000, the Swiss company ABB signed a contract to deliver not one, but two nuclear reactors to North Korea.
Here's the bit that should drain anyone of all faith in politicians: Donald Rumsfeld was an ABB board member at the time.
[1] https://www.swissinfo.ch/eng/rumsfeld-was-on-abb-board-durin...
https://www.ansto.gov.au/research/facilities/opal-multi-purp...
Follow the money. Christopher Pyne brokered this $50Bn deal to get re-elected (some boatbuilding would happen in SA) which blew out to $90Bn and rising.
Pyne immediately retired (got his pollie-perks-for-life golden ticket stamped) and then immediately became "employed" as a consult with the people profiting massively from this twisted deal against the public interest.
Pyne followed the same slimy playbook as Andrew Robb (4) who sneakily slipped the 99-year Darwin Port dodgy deal across the line, and then he got a free $880k "job" with them the day he left parliament shortly thereafter.
Borderline treasonous behavior is rife with our political elite (both sides stink badly)
https://www.macrobusiness.com.au/2016/04/would-you-employ-ch...
https://www.macrobusiness.com.au/2019/07/pyne-poisons-ey/
https://www.macrobusiness.com.au/2020/01/audit-office-torped...
(4) https://www.macrobusiness.com.au/2018/06/chinese-sell-andrew...
No, they're not. The sub deal with Australia was just part of a broader Pacific strategy to which Australia was central. Now that Australia sent the signal that it was to be a plugin in the US foreign policy rather than a country of its own, the entire strategy is dead and needs to be reworked. It's a major setback. It coming from the Nth sabotage of the French defense industry by the US is just the cherry on the top.
[1]: https://upload.wikimedia.org/wikipedia/commons/a/a2/Map_of_t...
I don't see anything in NPT that forbids the US and UK from supplying nuclear propulsion to Australia. They are not supplying nuclear warheads.
The reason the French are pissed are strictly commercial. But that is on Naval Group. As soon as the deal was struck between Australia and Naval Group, Naval Group were changing the terms, and were years behind on some of the earliest deliveries. Had the project been going well, Australia would not have spent political capital seeking this new deal.