There's no force involved in either case - workers can choose not to work at a union shop, just like they can choose* not to work under a non-compete.
*Until it becomes impossible to find a place without a non-compete.
Because (at least pre-Hobby-Lobby) laws have historically prioritized the rights of individuals over the rights of corporations. An employer which compels their employees to join a union is signing away the rights of those employees, while an employee who signs a non-compete is only signing away their own rights.
If the employer has a mandatory unionization agreement, you can't negotiate with them to change the terms of that.
They say no such thing. They say that companies can't form agreements to exclusively hire union workers. They outlaw essentially non-competes in the opposite direction. A non-compete agreement binds a worker to exclusivity with a company. The agreements outlawed by right-to-work laws bind companies to exclusivity with a union.
If you are thinking of "closed shops", companies where the union has a security agreement with the company and the union is free to deny membership to a person (thereby denying them employment), those were deemed illegal everywhere under US federal labor law.
A worker can't be forced to join a union even in the presence of such an agreement. The company simply agreed not to hire them if they don't join.
Taken to its logical conclusion, this semantic distinction could be used to justify eliminating just about any worker protection currently on the books.
That is why they are called right to work laws, and this is why the parent is making that observation, as NCA's are antithetical to right to work.
Maybe trying reading in to things a bit more rather then jumping to the wrong conclusion?
No, they mean employees have a choice to join a union or not (ie. pay union dues or not.)
In other words, they're a legal tool to weaken unions by reducing their membership and revenue.