one-way ticket has any potential problem when entering US?

My parents plan to visit me from China soon, I am shopping around for flight ticket, wonder if I should buy 2-way tickets(decide now on the return date) or I can buy one-way ticket for them so that we have flexibility in terms of when they will go back? Certainly if we buy 2 way, we can change the date if we want later, but I know you will have to pay some extra fee for changing date, I try to avoid that.

My concern for 1-way ticket is when entering US custom, will custom officer ask question why you don't buy 2 way? will they have doubt that you might not want to go back?

any similar experience?

Thanks,