It's not unfair. It's a voluntary transaction, in which both parties benefit. Both companies and workers have always behaved in a profit-maximizing way.
Simply put, “It's a voluntary transaction” can never be a sufficient argument for why something ought to be legal.
Eg. I'd rather be a slave than starving on the road, I'd rather be dead than in some particular irreversible painful medical condition.
The only actions which are ethically wrong in my book are hurting someone else or damaging / stealing their property. Incidentally the government breaks these rules legally everyday (eg. taxation, wars, spying on people).
And you would throw it all away because of some personal libertarian principle? This isn’t only about what you would like to be able to do in any given moment, it’s what this ability does to society as a whole. Your ability to someday be able to sell yourself into slavery has some value (for the principle of the thing, if nothing else), but it is certainly not worth the above-mentioned effects on society.
For instance, most people accept that for the same job and all things being equal you should pay a black/white person the same wage. If there was a job posting that said ‘we pay this skin colour X and this skin colour Y because according to our analysis people with darker skin tones are willing to accept a lower wage’ there would rightfully be outrage. It would be unfair, while being a voluntary transaction which is profit maximising and in which both parties benefit.
I know you aren’t arguing for that though - we accept as a society that we shouldn’t pay people different based on race, but lots of people do believe that we should pay people differently based on where they were born and what visa status they hold. I personally think this view will change over time as globalisation and remote working continues.