Brett wrote:

Why are 1-way tickets cheaper than round trip?

I think you meant to write that the other way around.

One way tickets are less expensive than full fare round trip tickets.
The difference is that there are discounts on round trip tickets, which
will often make them less expensive than one-way, depending on how early
you book your flights. They don't discount one-way, since there is no
reason to.

Can I use a round trip ticket as a 1 way?

Yes, you can do that. Just make sure it is the first flight that you
take, and not the second. If you miss the first flight, the rest of you
reservation will be canceled.

Is there any type of penalty for doing this?

Not normally.