U.S. federal regulations require that a B-2 visitor present evidence of
"financial arrangement" for eventual on-time departure from the U.S.
Quoting from the legacy Immigration and Naturalization Service Inspector's
Field Manual (the operational handbook used by port-of-entry immigration
inspectors in 2000; the current version is not publicly available):
Classification: B-2 Visitor for pleasure.
Documents required: Passport valid for a minimum of 6 months beyond the
period of admission, unless otherwise provided for or waived.
Nonimmigrant visa (B-2) unless waived.
Qualifications: Has a residence in a foreign country which the alien does
not intend to abandon. Subject to all nonimmigrant grounds of
inadmissibility. Intends to enter the U.S. for a temporary visit. Will
engage in legitimate activities relating to pleasure. Has made financial
arrangements to carry out the purpose of the visit to, and departure from,
the United States.
Terms of admission: Maximum admission is 1 year. If admissible, a B-2 is
generally admitted for 6 months.
Notations on I-94: B-2, (date to which admitted).
[End of quote]
What exactly is acceptable as evidence of "financial arrangments" depends on
the nationality of the visitor, the visitor's history of travel to the U.S.,
the mode of transportation for arrival into the U.S., etc. For many Chinese
visitors flying from China to the U.S., round-trip tickets may be required.
Without round-trip tickets, the visitor could be (but is not always
necessarily) denied entry into the U.S.
If the U.S. immigration officer does deny a visitor entry at an airport and
does not want to detain the visitor, that visitor is turned over to the
carrier (airline, bus, or train company) that brought him to the U.S. A
fine may be levied against the carrier, and the carrier must assume the cost
of transporting the deported visitor back to the country of embarkation.
Therefore some airlines or transportation companies may deny boarding to
passengers who have not purchased round-trip tickets if they feel that the
passenger might be refused entry into the U.S. by Customs and Border
Protection immigration inspector.
So, without round-trip tickets, some people are denied entry into the U.S.,
some have trouble just trying to board their flights, bus, or train, and of
course some make it all the way to the U.S. and through immigration
inspection with no problem.