I think it is what the airlines call "throw away ticketing", it is discouraged, but as stated, they will credit you for the miles you actually flew and there will be no retribution for not using the return.
However, it is quite often against the airlines rules and if you did it very often they might go back and charge you the one-way fares. The are looking for habits, not single occurances.
There are a lot of reasons why you did not fly the return. The airline knows this and is not overly concerned with it.