I concur with other answers with respect to legitimacy of the ticket. However you should watch for visa requirements which may vary from what you'd expect.

For example, I used to do something similar to what you are describing, originating in Thailand and flying into the US and back. The return leg on the ticket was then essentially a one-way into Thailand. Although Thailand is visa-free for US citizens, the airline's conditions technically required a visa for one way trips into Thailand. On more than one occasion that was actually enforced (I had a visa so no problem).

I think you are unlikely to encounter this between the US and Europe, but it would make sense to at least research it and make sure you're not outsmarting yourself.