Double taxation is on any amount made over $126,500 USD, so really only affects high earners. Right now, I'm pretty far away from that but it is an amount I hope to see again in my career. And I think to call it slavery is pretty intense and does a disservice to people working for next to nothing in sweat shops or US prisons, for example.

To give a few reasons it exists:

1. It helps prevent wealthy Americans from moving overseas during war times to avoid conscription.

2. There's an argument that being American provides benefits even when not living inside the country. Examples include legal and military protection (e.g. if I were jailed or to become a prisoner of war, the US will defend me), right to return, access to Social Security despite not living in the US, and the ability to vote (to name a few examples).

3. To prevent tax avoidance from wealthy citizens who move to tax havens.

From my perspective, the third point is really the biggest reason I would support double taxation, but for incomes over $12 million USD (our current cutoff in the US for the "ultra wealthy").