and lots of people took trips to the Soviet Union. Well, not lots, but certainly more than a handful. Depending on timing, it wasn't that big of a deal to visit behind the Iron Curtain. They liked American dollars just like everybody else. The "wall", like all bad walls, was designed to keep people in, not out.
The interesting thing is that Oswald didn't take a trip, but actually
defected and came back with a Russian wife. This wasn't a student going behind the curtain for a couple of weeks, but rather someone going to live there.
On the point though whilst Oswald has certain ties to these elements, it really wasn't that unnatural back in the Cold War- especially as he was someone who worked within the U2 program.