Well, that automatically follows from the unspecified nature of the alleged designer. As soon as we have the designer, I don't deny that we can learn about his methods and motivations and make inferences on what we should resp. shouldn't observe.
However, as long as this designer remains unidentified we can't make any meaningful inferences.
And to be honest, I haven't seen that any IDer has in any way identified this alleged designer or ascribed any concrete properties to him.
You're basically demanding proof of a designer before we could possibly hypothesize design in any given case. That's very convenient to the anti-ID cause, but it's not logically necessary for the proposal and testing of the hypothesis in a specific case.
And thus, again, I'll bring up my example of the insulin-producing bacterium. We know that ID is the correct answer for that case (and we know of no other insulin-producing bacteria), so it provides a good basis on which to test the claims being made for science in this debate.
First, is it possible for science to get the correct answer at all? I tend to think the answer is yes -- the specificity of the gene in an otherwise unremarkable bacterium would seem to be a good first indication that there is something interesting going on.
Second, addressing your comment, is it possible for science to get the right answer without having first to discover the engineer who did it? Again, I think the answer may be yes, because (based on my reading of the techniques used to create insulin-producing bacteria) there are certain things that must be done to make the gene actually express insulin -- and those modifications would be a marker for an intelligent agent.
Now, it may well be more difficult to do that if a specific "design event" were followed by long time periods in which the property was then subjected to random mutation.