Why must a russian citizen buy round trip ticket to visit me in the USA?

I ,meet a lady online,she said she got her medical pass, went to get her tourist visa to come and visit me and was denied because she only bought a one way ticket,now for her to visit me she needs 600 more dollars for a round trip ticket,WHY?????

It's evidence to the US immigration authorities that she doesn't intend to stay as an Illegal immigrant. If she has proof (in this case, the round trip ticket) that she intends to go home after her visit, then that will help her case.