Just because it doesn't always work doesn't mean anything. There are also minumum speed laws but no one expects them to be enforced in a traffic jam.
I’m not talking about a traffic jam. I’m talking about a full highway at normal highway speed. By full highway, I mean all lanes contain cars spaced approximately “assured clear distance” or a little less, and by normal highway speed I mean 60-70mph on a two-lane divided highway. “Left lane for passing only” does NOT work under those circumstances, which are quite common in many areas I frequent. The highway runs just fine as long as we don’t have any idiots in either lane moving significantly below or above that speed range. The tailgating, lane-weaving idiots clearly do expect “Left lane for passing” to be enforced, and get quite irate when it (invariably) is not.