They never promised that. They merely promised that they wouldn't break into the house, not that nobody would.
I also haven't made up my mind one way or the other about any level of US/UK involvement on behalf of Ukraine. Reality is they had about 36 hours after Ukrainians booted Moscow's puppet to make Crimea too bothersome for Putin to just walk in, but none of those gov'ts had any political resources for playing Putin's brand of game.
But I think you're wrong about no obligations to enforce Ukraine's territorial integrity. The US and UK obviously were no threat to Ukraine at that time - the only reason they were signatories was to provide support vs Russia. None of us can know the particulars of the talks leading up to the agreement. And sure, people can argue about the fine print or admonish Ukraine for not hiring better lawyers before letting US/UK/Russia talk them into surrendering their amazing nuclear arsenal.
I think we all can reasonably understand that Ukraine was led to believe they'd get some heavy lifting on their behalf in exchange for giving up a nuclear capability greater than all other nations except US and Russia. And so we are now at this entirely predictable crossroads, and the US and UK get to either man up (in some fashion) or weasel out and take their deserved lumps for selling their crippled buddy down the river.