Respectfully, I disagree, that's just progressive taxation and it's true of every other aspect of our tax system. Someone who pays 10X or 100X more taxes than me isn't getting 100X the value out of our roads, bridges, or the army.
It's fair IMO because of the marginal utility of money. If you make $50,000 per year, $10,000 means a lot. If you make $10M, $10K is a rounding error or a fun weekend in Vegas. A progressive tax system is IMO flat when plotted against the marginal utility of the value accrued. Think of it in terms of burgers. I require 3 burgers a day to live. I get paid 3. If you try and take 1, I go hungry. That burger is worth a lot to me. On the other hand if I get paid 10 burgers, and you try and take 1, it means nothing to me. That burger is worth a lot less because I'm already so full. Same is true in the abstract in terms of dollars.
The next most popular choice are NSOs, which are taxed as ordinary income when exercised on the difference between the strike price and the fair market value -- and you do in fact pay Social Security and Medicare taxes. Once you hold the stock, it's no longer compensation, it's an asset you've bought and paid for.
Third most popular are ISOs which do have some preferential treatment, although it's rare to be issued ISOs at anything other than a very early stage company, and at that point, you're likely to exercise early, and then once again, you've bought and paid for an asset and the it's no longer your compensation.
Founder shares are also assets and not compensation, and taxed as such, assuming you file your 83(b) election in a timely fashion.
I think it's fair to say that the rare issuance of ISOs notwithstanding, stock compensation is taxed as ordinary income and is exposed to social security and medicare taxes.
Finally -- and most importantly, IMO -- social security tax stops accruing after you've earned $142,800USD. Chances are if you're earning stock compensation in the Bay Area, that's covered by your base pay plus bonuses, if applicable.
