It's not the U-233 itself that emits gamma, but the U-232 it tends to be contaminated with. This can be avoided by chemically separating protactinium during the decay chain from thorium, but the U-232 is often considered desirable as a anti-proliferation measure.

By using liquid fuel and transmuting in place, you never handle U-233, contaminated or not. After reactor startup (which requires a good neutron source), you just keep feeding more thorium to the reactor and removing fission products.