Originally Posted by velonomad
I have seen that sign before in other places. it is intended to prevent motorists from using the bike lane as a curb lane to pass other motorists on the right when making a right turn in stopped traffic.
This is the most plausible explanation. The city needs a bigger sign, so they can explain it as lucidly as you have done here.
I will see if I can get a traffic engineer from the city/county to read this thread. If I wanted to be scrupulous about not committing a violation at that intersection, I'd avoid making any turns, either as a bicyclist or a motorist. The sign fails to solve whatever problem prompted it to be posted there.