The alternative is that you're stuck going 10+ mph under the limit because of a left lane hog.
Banning passing on the right only works if keep-right-except-to-pass laws exist and are thoroughly enforced. Most states in the USA don't have keep-right laws, and those that do, it's never enforced, so many people don't even know it's actually the law.
EDIT: US-26 westbound immediately outside downtown Portland is notorious for left lane campers. The limit is 50 mph, but without fail, there's some moron going 40 mph in the left lane. You wanna go slow? Fine, but do it in the right lane where you belong.
AFAIK all US states have some form of "keep right" or "slow poke" laws in various forms that distill down to keep right, except to pass.
So the laws do exist, they just are never enforced. Even when said left lane campers cruise by state patrol, I never see them stopped. Hell, I rarely see them stop people going 15+ over the limit either so not actually sure what they're doing.
Its not against the law in any US state (a quick search seems to back this up) to pass on the right. With one huge gotcha, it must be "safe" defined in various ways.
OTOH, most states have a stay right except to pass, slower traffic keep right laws.
Which means, that unless the person to your right is weaving through traffic, driving on the shoulder, or a few other bits of unsafe behavior, if someone passes you on the right your likely the one violating the law by not moving right when your not actively overtaking/passing someone.
There is not a single state in the USA where using the right lane to pass slower traffic in the left/center lanes is illegal. It is allowed everywhere.
