It's not merely forbidding retaliation - it is an outright assault on the liberal principle of freedom to associate and transact.
You use words like retaliation as if it were violence or force. But abstention to continue an economic relationship is quite simply a natural right. Abstaining from a transaction/interaction/relationship is the most basic right that exists. Denying that, denying "exit" eliminates any notion of freedom from an interaction. It is likewise your right to not be in any romantic relationship that you find does not suit you. The logic at play, if applied to relationships other than economic, would amount to requiring someone to remain married to someone who had broken their promises. That's not argumentum ad absurdum, it's an application of the core principle underlying this, applied in different spheres.
If there are obligations created through contract, those must be honoured, so if someone is fired for a breach, but work was performed for which payment is due, that payment is ... due. But an ongoing relationship is not an entitlement, it is a mutually consenting and mutually beneficial agreement.
Do I think this sort of practice is a dick-move? Yes. I would probably boycott such a firm, and do what I could to sully their reputation for such behaviour. But that does not mean I would seek to legislatively limit their right to end an economic relationship, even if I think it's for dickish reasons.
Attempts to coerce parties into engaging in contracts, or prohibiting them from contracts is a fundamental denial of social harmony - done in the name of justice. But it comes from a notion that a third-party can know what is best for two parties engaged in a transaction, or that that third-party can have the moral authority to compel a transaction, or prohibit one. And I deny that on ethical grounds, and suggest that, on consequential grounds, the cure proposed is far, far worse (in the long run) than the disease.