To play devil's advocate, why would Intel spend money to allow their competitor to leverage speedups from their compiler?

Because in many countries, including US, it's illegal to leverage one product to ensure unfair market advantage for your other products, and this is very close to that...

Intel spent money to prevent their competitor from leveraging speedups from their compiler. It would have been less work for them to simply use feature detection on AMD chips, since Intel already has to implement feature detection for their own chips.

It took slightly more effort for them to put in the anti-optimization feature than to do the feature detection correctly.

So the answer to your question is "they don't need to", but your question has nothing to do with the problem.

Because it would benefit the user of the compiler, their customer.

Wow. Are you sure? How do you know this?

CentaurTechnologies ran benchmarks against ICC with the CPUID set to the default and then with it set to the name of an Intel chip with equivalent CPU flags.

Intel's response was roughly "We don't trust CPU flags, so we have kernels for each specific Intel chip, and a generic kernel for non-Intel chips"

Luckily, from Agner's CPU blog linked above:

"[Update 2019:] The newest versions of Gnu and Clang C++ compilers are now optimizing better than the Intel compiler in my tests."

Good reason to avoid Intel compiler even more.

Newest ICC is an LLVM frontend anyway. Maybe some special sauce, maybe not.

The link in the post above is a reasonable intro: https://www.agner.org/optimize/blog/read.php?i=49

Thank you!