Legally? Sure.
Ethically? No.
> I'm sure the founders feel it was a good business decision.
The next founders who contemplate pulling a cowardly stunt like this shouldn't feel it to be a good business decision.
As it stands, this reflects terribly YC and its startups as a whole.
Hiring for first stage startups is critically important -- ie. it can make or break the company.
If the OP wasn't going to work out for any reason, then it was appropriate to cut him loose as soon as possible, instead of dragging it out.
One (not primary, but fairly clear) source:
http://www.cslaw.com/our-practice/san-francisco/employment-c...
Googling "california at-will good faith" will unearth much more.
Edit: more detail, including that this isn't specific to explicit employment contracts. Also, I'm in no way saying this case would fall under this, only that at-will isn't absolute in CA, even beyond the normal discrimination boundaries.
http://legal-dictionary.thefreedictionary.com/Breach+of+an+I...
Alupis is absolutely right, startups are volatile and it could have been worse. That this was legal means this was probably the only right option for the founders.
Making hires at an early stage is incredibly important, imagine this startup has only a few 100k in the bank. They can hire maybe two to three persons, and that's hoping they'll hit on more funding later in the year.
Imagine you were a founder of that company, and you had hire this COO, and then something changes, or a pivot comes through just when the COO is flown in. Your investors all say you should discharge him, so they can hire someone that might save the company.
How can you even suggest this is unethical? This guy goes work at a startup in a state where it is legal to fire someone at will.
All legal.
Maybe this business succeeds, maybe they fail (they are a startup, and most startups don't succeed).
But, the OP will always need a job. And quite frankly, getting upset over a termination and coming online and bringing major flak to the business is a sure-fire way to get the OP rejected out of any HM's stack if they know what they are doing. Why would I want to hire someone who has a history of flaming previous employers? It doesn't matter the reason they flamed the previous employers, it's the fact that they did it at all. That goes double for anyone who is trying to get hired into top executive positions, such as COO.
It's not personal, it's business... and if the OP can't distinguish between the two, then he's not ready for top level executive positions.
California has At-Will employment -- so they can hire and fire you for any reason whenever they want. "You weren't a good fit", "You didn't think Big enough", "You didn't produce enough", etc etc etc.
Coming on here and ranting and trying to bring some action against the startup is not a good way to handle this. You feel wronged -- I'm sure the founders feel it was a good business decision.
Sorry this happened to you... but Startups are very volatile. It could have been worse.