I'll start by noting that the two instruments are not directly comparable, as they have somewhat different functions, and fairly different formats. This cut sheet compares the two:
http://images.pearsonclinical.com/images/assets/celfpreschool-2/PLS-5-CELF-5_Comparison.pdfSecondly, the purpose of this kind of evaluation is, in fact, to identify deficits in need of remediation. It is not to define the above-average strengths of the child. This is why the tests are all designed to prove that he either is or is not delayed in some area. That we often find that children are advanced in some area is kind of a handy side effect, but still incidental. Once children reach school-age, aims start to change a bit, depending on how a school system interprets "free and appropriate education".
Thirdly, he was not administered higher-level items on the CELF, most likely because there are two forms, one for age 3-4 and one for age 5-6. This also may be the source of ceiling effects. The PLS actually is normed through age 7, so there was quite a bit more headroom there (and even then, he didn't reach a discontinue criterion, as you note).
The BDI-II he was administered was the screening version only, or you would have received a true standard score, not an estimated score. Again, this is designed primarily to distinguish between children in need of services, and those not so. The screening version normally generates only cut scores, not standard scores, and would not have higher-level items on it.
And "above age appropriate" is by no means a bad thing. I put statements like that in my reports all the time ("above age expectations" usually), as a simple observation, not as a value judgement. Now, the question of whether they are antagonistic to dealing with possible giftedness is another question altogether, which you are in a better position to assess than any of us are. But I don't think there is clear evidence for bias in the evaluation results themselves, or in the way they conducted it.
The bottom line is, your DS's evaluation was entirely appropriate for its purpose--which does not include identifying him as gifted. It does identify him as having a communication disorder, though, in the area of pragmatic language. You may find that the goals change after reaching school age, when academics become part of the equation. As to additional testing, it is not that they are likely to do a bad IQ test, it's more that this is not part of what their mandate is at this age.
