"Opt-in collective bargaining" sounds suspiciously like "right to work" laws. I don't think you meant it like that, but it sounds like that.
In case someone isn't aware, a union shop, has a union that negotiates a contract with management. It sets wages, benefits, safety, all sorts of stuff. Most importantly, it negotiates for all workers at the shop. The reason why it negotiates for all workers (or more precisely all workers that qualify for union (e.g. all pilots, all janitors, all machinists, etc.) is that 1) it increases bargaining power, and 2) it simplifies things for the company. As a result, in a union shop, if you work there in a capacity that is union eligible, you're compelled to pay dues to union for the purposes of contract negotiating. (Dues for other purposes are not compelled.) Everyone benefits from the contract. Everyone pays to negotiate said contract.
"Right to work" laws say that you can't be compelled to pay dues to negotiate contracts, and yet you still benefit from the contracts. It's a legal carve out for free riders, with the purpose to starve unions of money and thus negotiating power. This isn't a side effect of this policy, it's its purpose. That's why you find pro-management groups and politicians advocating for these laws, while denigrating unions.