Not paying out the 2 weeks would be sort of a jerk move. But simply cutting a check and saying "It's been good having you" as they show you the door is perfectly reasonable in some circumstances.
The risk isn't that you'll call them a jerk, it is that when your new job falls through, your old job can no longer credibly claim that you left voluntarily because they moved your quit date. You were fired. So you'll be able to claim unemployment and the business' unemployment insurance premiums could rise.
It's bound to be cheaper to pay the two weeks than to pay the increase in unemployment insurance premiums for every remaining employee in perpetuity. In that sense, it's short-sighted to not pay you through your notice period.
I'm pretty sure they can accept your resignation effective immediately and it not be a firing. Post dating it is a nicety you offered and they declined.
Of course you can be terminated at any time, but that would not be your resignation anymore. You are an at-will employee at all times. You can be fired, or you can quit. But let's not conflate the two.
I'm pretty sure this lawyer knows a thing or two. And anyway, nothing is open-and-shut. Giving notice isn't "cause for termination" by any reasonable standard.
We were talking about risks. You can be pretty sure, and then a judge rules against you. Are you willing to take that risk? (Sure, maybe you know a judge. Or you live in a state with fewer worker protections. IDK, but I'm not making this up, and laws vary from state to state.)
The financial risk is small, can often be less than the cost of paying the salary over those 2 weeks. The risk the person is going to lawyer up is also fairly small, but at that point you just settle, you've probably lost the gamble at that point. If let it go to court, you're definitely going to lose on a financial perspective.
Either way, it's not a hill I'd die upon, my policy has always been to pay it out because it's just the right way of acting from an ethics perspective. I think the link you posted makes sense in our current world, but it's also using CA as an example and my gut tells me where I live is not as progressive; along with ~half or more of the US.
It is a courtesy to give notice, and one must generally be prepared to extend courtesies in order to receive them in kind. You sound like a person who understands all of this. It is certainly going to vary from state to state.
As far as risks go, increasing UI premiums is one risk; it might be easy to invent a valid cause for immediate termination when someone gives notice, but also quite transparent in terms of ethics as you say. Concretely, that would also be leaving employers at a greater risk that word gets around with the remaining employees, and then you likely won't see employees giving notice anymore, or affecting morale of the remaining employees, or what else.
To be fair, I'm pretty sure that I won't find the precedent I'm looking for in my own state either, but the link above is interesting: if your company has a policy that employees must give notice to quit, whether it was legally enforceable or not, with a termination after notice is given that policy could be used as evidence against you!
I guess it's a good idea to have a firm grasp of the law and review your own company's policies regularly, to be sure they align with the law! For another example in the same vein, I thought that only California made non-competes illegal but there are at least two other states, and it may be illegal to try to enforce a non-compete at a federal level soon.
