I exchanged a few emails with IK Multimedia yesterday. They explained that if any other audio app is running, they disable the "ultra-low" latency option and use whatever latency setting (technically, the audio buffer size) the other app requested. The problem is that my apps don't set the buffer size at all, they just leave it at the system default, so there's no benefit to SampleTank disabling this option in this case.
I recommended to IK Multimedia that they only disable this option if the other app has actually set a buffer size; if the other app doesn't specify a buffer size, then they should go ahead and set it. If their app can't tell if another app has set the buffer size (there might not be a reliable way to determine this), they could go ahead and enable the option anyway, and update their hint text to say, "Set to Low if you hear clicks or pops in this or other audio apps."
They said they'd consider my feedback. If you like, you could contact them with the same feedback and maybe that would give it more weight.