It is not federally 'illegal' in the US, but prohibited in some manner in most states. Although it isn't about picking up a hitchhiker but rather the person asking for the ride. Considering everyone has cell phones taped to their ears, these days you could at least call the Sherrif and tell them someone is stranded on the side of the road.
Alabama
People are prohibited from standing in the roadway to solicit a ride.
Alaska
Hitchhiking may be legal.
Arizona
Pedestrians are supposed to walk on sidewalks if available, not along the roadway. If there are no sidewalks, they have to walk facing towards oncoming traffic, and are prohibited from standing in the roadway to solicit a ride.
Arkansas
It is illegal to hitchhike in Arkansas.
California
People are prohibited from standing in the roadway to solicit a ride.
Colorado
Colorado has a complex system of rules regarding pedestrians on or near a roadway. Essentially, they are required to walk as far from the road as possible if there is no sidewalk, or on the sidewalk if there is. They cannot solicit rides on state roads, and local municipalities can pass their own laws regarding hitchhiking on local roads. If someone does stop to pick up a pedestrian, they have to pull as far off the road as possible so as not to impede other traffic.
Connecticut
Soliciting a ride from anyone other than a cab or bus while in the road is an infraction punishable by a $35 fine except in the case of an accident or emergency. However, soliciting a ride from the shoulder is acceptable.
Delaware
One cannot solicit a ride or employment or money while standing in the roadway.
Florida
One cannot solicit a ride or employment or money while standing in the roadway.
Georgia
One cannot solicit a ride while standing in the roadway.
Hawaii
One cannot solicit rides from the roadway.
Idaho
One cannot stand on a highway to solicit a ride.
Illinois
One cannot solicit rides from the roadway.
Indiana
One cannot solicit rides from the roadway unless there is an emergency.
Iowa
One cannot solicit rides from the roadway, but they are allowed to do so if not standing in the road.
Kansas
One cannot solicit rides from the roadway.
Kentucky
One cannot solicit rides from the roadway.
Louisiana
One cannot solicit rides from the roadway.
Maine
Hitchhiking is forbidden
Maryland
One cannot solicit rides from the roadway.
Massachusetts
One who solicits a ride on the public roadway can be fined $50
Michigan
Generally pedestrians cannot interfere with traffic flow, but there is no express prohibition against hitchhiking at a state level.
Minnesota
One cannot solicit rides from the roadway.
Mississippi
One cannot solicit rides from the roadway.
Missouri
As long as the hitchhiker does not impede the flow of traffic or interfere with the traffic it is permitted.
Montana
One cannot solicit rides, employment, or money from the roadway.
Nebraska
One cannot stand in a roadway for the purpose of soliciting a ride, employment, contributions, or business from the occupant of any vehicle.
Nevada
A person shall not stand in a highway to solicit a ride or any business from the driver or any occupant of a vehickle.
New Hampshire
One cannot solicit rides from the roadway.
New Jersey
One cannot solicit rides from the roadway.
New Mexico
One cannot solicit rides from the roadway.
New York
One cannot solicit rides from the roadway.
North Carolina
One cannot solicit rides from the roadway.
North Dakota
One cannot solicit rides from the roadway.
Ohio
One cannot solicit rides from the roadway.
Oklahoma
Oklahoma only prohibits hitchhiking on turnpikes.
Oregon
Prohibits hitchhiking.
Pennsylvania
Hitchhiking prohibited.
Rhode Island
One cannot solicit rides from the roadway.
South Carolina
One cannot solicit rides from the roadway.
South Dakota
Likely permitted.
Tennessee
Hitchhiking prohibited.
Texas
One cannot solicit rides from the roadway.
Utah
One cannot solicit rides from the roadway.
Vermont
One cannot solicit rides from the roadway.
Virginia
Pedestrians shall not stand or stop in any roadway for the purpose of soliciting rides.
Washington
Hitchhiking prohibited.
West Virginia
One cannot solicit rides from the roadway.
Wisconsin
One cannot solicit rides from the roadway.
Wyoming
One cannot solicit rides from the roadway.