Because the effect of high output impedance isn't the same for all phones, and with some phones the change in frequency response can even make them sound better. When I tried the Tera Player I felt it was a great pairing for the Sennheiser IE800 and the FitEar TOGO 334, because its bass roll-off with low impedance phones made these bass enhanced IEMs sound more balanced. But on the other hand my already rather thin and bass light Final Audio FI-BA-SS ended up sounding completely weird.
Bottom line, please don't mistake the meaning of Charles' unloaded RMAA measurements, they say actually very little about how the Tera Player will behave under load with your phones, especially if you're using low impedance IEMs.
Btw, shotgunshane just answered a similar question in the AK100 thread:
It depends on the impedance curve of the iem. Based on most of the BA and multi-BA impedance graphs I've seen I'd say yes, you'll get bass roll off. Sometime moderate, sometimes much greater. You can get a lot of variance in treble response as well.
Edit: it's worth noting that sometimes the change can be agreeable. In the case if the tg334 and w4, some like adding 33ohms or so, to get them closer to a reference frequency response. But others, such as the UE TF10 and many Shure IEMs just sound plain odd or bad with added impedance. That's one of the reasons less than 1 ohms for amplifiers is recommended.