In that case taxes should be negative and there could be other mechanism transposing the slope, like a disability, old age, kids, doing community service (anything falling under state employment for that matter).
When it comes down to it, if you are willing to strip down unnecessary bureaucracy what are now taxes could substitute any governmental mechanism involving money.
At best the thing would be tied to consumption, not income, property, profit or something else. Of course that would require a drastic change in the money system as well, which brings us back to bitcoin...
Why tax them? Well, if they're using services that cost money, shouldn't they pay for those services just like everyone else?
The contrary argument is: the higher the tax on the rich, the more inclined they are to reduce their tax burden by employing more people.
Both are speculation, the argument I propose might seem to be backed up by the data.