Earlier quoted context omitted.
If anything, wouldn't airlines be happy if you already paid for the flight and miss it? Especially if they overbook.
Internationally it is common for a roundtrip to be cheaper than a one-way. No, not cheaper than two one-ways. Cheaper than one one-way.
They tend to tier things so business travelers will pay more, so there may be fare rules that increase the cost dramatically the closer you get to departure date, or charge more if you do not stay a certain number of days (I've heard one week as well as requiring a 'Saturday stayover')
Of course business travelers are the ones most likely to research and plan out how to get around these, so they might book overlapping tickets pointing opposite directions and interleave them. Airlines get upset about this, but they also know that if they try to crack down that these travelers will just take their (substantial) business elsewhere.