We are also the only profession that is required by law in most states to give our time free of charge to the indigent. When I was in law school, that was called taking property without just compensation. The state calls it a condition of abtaining a license to practice.
How many "professions" have an exclusive charter, (monopoly) in dealing with government agencies?
Well, I wouldn't say only. Don't forget, psychology majors in their final year are required by most states to do community service at state-operated clinics before they receive their doctorates.
Many licensed psychologists continue to work in these clinics free of charge "after hours," supervising psychology students and assisting on cases. This is done on a voluntary basis -- these doctors are not compelled by state law to contribute their time.
Perhaps if the lawyers were as generous with their time, the state would not feel compelled to force them into pro-bono work against their will.
Personally, I don't believe the state has any business regulating the off-duty hours of licensed professionals, but what do I know. I didn't write the law.