Unions in the US (with some small exceptions) have to negotiate better working conditions which benefit all workers, regardless of whether they are a part of the union or not [1]. If there was no mandatory fee, then unions would likely suffer from the free rider problem and be suboptimally funded.
Of course, this doesn't mean that all unions' fees are justifiable for the value they produce, nor does it mean that all unions which impose mandatory fees improve working conditions for all workers. But I think it's hyperbole to call mandatory fees "rent-seeking" when there might be a perfectly rational explanation in some instances.
Mandatory fees are rent-seeking either way, it's just possible that imposing the rent is justified.
Off-hand, I'm not convinced. I'm leaning more and more against systems designed to prevent free riders—our culture is so primed to value "fairness" that we're willing to impose significant net harm to achieve it. I usually think about this in terms of public policy (free public transport, subsidizing the unemployed/homeless/etc...) but I think it applies here too.
I would not be surprised if a union that had to justify its dues to workers would be more effective for workers than one with mandatory dues because there would be more direct pressure for the union to be transparent and effective. I am not entirely confident in this though, in large part because I don't understand the politics around unions in practice! Mandatory dues do seem like a viable point of leverage against union-busting attempts from management, which could be a more important effect.
You are not using rent in the conventional, economic sense. Rent is specifically a payment made to an manufacturer of goods or services that is higher than what is necessary for that manufacturer to bring those goods into existence; (i.e. higher than the profit margin that would have incentivized that manufacturer to produce the good in the first place).
For a trivial example, if a union makes no profit and its actions provide more utility to its employees than their mandatory payments, then it is not receiving any economic rent.
> I'm leaning more and more against systems designed to prevent free riders—our culture is so primed to value "fairness" that we're willing to impose significant net harm to achieve it.
I'm not sure if you fully understand what free rider problems are. The whole point of preventing free riders is that by imposing a tax or mandatory payment, you increase net utility. If imposing a tax truly imposed significant net harm, then the problem wouldn't be a free rider problem by definition!
Free rider problems are really coordination problems. Essentially, there are situations where if everyone cooperated, then everyone would be happier. But there is no incentive for any one person to cooperate because they can choose to defect, yet still receive the benefits of cooperation.
> free public transport, subsidizing the unemployed/homeless/etc...
First of all, these are not solutions to free rider problems. Second of all, it is not clear that they impose significant net harm. For example, you might claim that welfare reduces the incentive to work and or educate themselves. This is the typical conservative market-fundamentalist argument.
But practically speaking, humans need to take care of basic needs before they can start working or learn. The positive effect of nutrition, taking care of basic needs, etc. might overwhelm the negative effects from reduced incentive to work. So you need a lot of sophisticated analysis on real data before you can claim that these policies actually impose net harm.
Of course, this doesn't mean that all unions' fees are justifiable for the value they produce, nor does it mean that all unions which impose mandatory fees improve working conditions for all workers. But I think it's hyperbole to call mandatory fees "rent-seeking" when there might be a perfectly rational explanation in some instances.
[1] https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/em....