The contemporary socialist account of capitalist exploitation has less to do with capitalism than you might think. Many of today’s socialists grant that the labor theory of value should be abandoned and have updated their view of exploitation accordingly. The simplified version of the new account goes like this: under capitalism, a worker must either work for a capitalist or starve. So workers are effectively forced to accept capitalist employment and capitalist employers take advantage of their vulnerability in various ways.
Note, however, that the same issue could arise under socialism. Suppose we have a socialist regime with collectivized workplaces. Here a worker could find herself in a situation where she must work for a cooperative or starve. So she is effectively forced to accept socialist employment.
Socialists might reply that they’d supplement workplace collectivization with redistribution to avoid this sort of situation. Fair enough, but if redistribution can save workers from exploitation under socialism, it can save them from exploitation under capitalism.
More generally, the socialist account of exploitation only gets socialists to the conclusion that capitalist wage labor is sometimes exploitative; it cannot justify the much stronger claim that capitalist wage labor is exploitative by its very nature.
Consider the following case. Al has a $2 billion net worth and loves playing baseball. Every team in Major League Baseball offers him $30 million to play shortstop for them next season. Al carefully considers each offer and decides he’d like to live in LA for the year and signs with the Dodgers. There’s no exploitation here.
Of course, this is an exceptional case, but it shows that it’s not true that capitalist wage labor must be exploitative. Plus, as I noted, there are policies that we can implement to reduce exploitation, such as a UBI that gives workers more power to exit bad jobs.
Lastly, I’ll point out that the socialist account of exploitation isn’t about economic relationships in particular (as some socialists have recognized). Judith Thomson, for instance, argues that your right of bodily autonomy entitles you to withhold life-saving help that requires the use of your body. She gives the example of Henry Fonda having the right not to touch a patient’s “fevered brow” to save their life. Suppose Fonda offers to touch the patient’s fevered brow to save their life but only if the patient does something that will be very bad for them—say, get a divorce. This counts as exploitation on the account I’m considering, but it has nothing to do with capitalism.
Of course, my argument here is consistent with the claim that capitalism is sometimes exploitative on the socialist account of exploitation. My point is simply that socialists often speak as though exploitation is something unique to capitalism, but it clearly isn’t.
Moreover, the situation is symmetric between employees and employers: you could similarly argue that employers have to keep their companies afloat to survive, so they are forced to hire employees or starve.
This means that neither party has that much more bargaining power than the other.