Not true. I have been experimenting with AirAsia booking website.
On the first page of price inquiry, you must click either roundtrip or one-way.
If you inquire roundtrip fare, the search will return with a list of each segment price. To fininalize the purchase you need to select all segments. It does not allow you to buy only one segment.
If you inquire one-way fare, the search will return with a much higher price for one-way segment. The price is close to roundtrip price.