I can kind of see the logic. It's certainly possible for an adult to learn to speak another language indistinguishably from a native speaker, but in practice there's a surprisingly large gap between that and a person who can speak fluently on complex subjects with no fear of being misunderstood, but makes occasional minor errors--or even usages which are grammatically correct, but sound stilted--that betray them as non-native.
There's a big jump between 99.9% and 100%, is what I'm saying, and if you want a really professional translation job, you're not going to settle for 99.9%. And the thing is, you can't tell the difference between 99.9% fluent and 100% fluent without being 100% fluent. So if you want to be sure you're getting 100% fluency, a native speaker is your best bet.
There's a big jump between 99.9% and 100%, is what I'm saying, and if you want a really professional translation job, you're not going to settle for 99.9%. And the thing is, you can't tell the difference between 99.9% fluent and 100% fluent without being 100% fluent. So if you want to be sure you're getting 100% fluency, a native speaker is your best bet.