Since you mentioned the US, I feel it's important to bring up the Americans with Disabilities Act (ADA). Among many other things, the ADA has this to say on the subject:
Advisory 404.2.7 Door and Gate Hardware. Door hardware that can be operated with a closed fist or a loose grip accommodates the greatest range of users. Hardware that requires simultaneous hand and finger movements require greater dexterity and coordination, and is not recommended.
In other words, the ADA recommends against door knobs, but does not require lever handles.
Do note that the ADA does not apply to all doors in the country (private residences which are not also used for business purposes do not need to adhere to the ADA, for example), and the ADA only applies to things built (or modified) after the ADA went into effect (July 1992), so an old building that hasn't been updated wouldn't necessarily be subject to the ADA standards.