Hard to explain... but just because a trait is commonly found in certain colors doesn't mean it's exclusive to that color.
Yes, tarheels and toe penciling occur in most bi-colors, but they're not really a "bi-color trait" in the sense that they don't help define the dog as a bi-color. And thus they can occur in other colors as well.
They appear to be inherited separately from general color and pattern. Either they are linked somehow to the colors for sable and bi-color and are inherited together, and unable to link with B&T genes or (more likely IMO) the genes that cause the B&T color do so by prohibiting melanin expression on the legs, which would prohibit harheels and toe penciling too and explains why B&T dogs never have tarheels or toe penciling, while other colors can. Blacks might carry tarheels and toe penciling as well.. we just can't see it.