I would argue that this is actually a big plus. If you just fly round trips everywhere then it probably doesn't matter, but booking one-way tickets are very useful for some people and some travel patterns. For things like open jaw tickets, using ground transportation one way, mixing airlines such as LCCs or on other alliances, one-way awards are great and I've used them a lot on UA awards. Besides, cash tickets sometimes are cheap one way but expensive on the return, especially around holidays and for shorter trips (less common but still possible for intercontinental flights).

All that said, offering one-way awards really doesn't cost TG anything (if they manage inventory sensibly, which is a big if). When Delta started offering one-way awards several years ago the same thing or worse happened with prices, with some awards costing a million miles now.