This thread says Citi requested one: http://ficoforums.myfico.com/t5/Credit-Cards/Discover-More-Approved-Citi-thank-you-4506t/m-p/1372447...
That's interesting. Looks like this was during the application process, rather than after freezing the account of an existing customer. Personally doing it at app time makes much more sense to me! Still pretty intrusive of course.
Amex charge card users run a significantly higher montly multiple than Visa or MC users. Just from that pov Amex would be expected to do more FRs.
Also, Amex has a much deeper FR history. Probably enough to be able to correlate card usage with tax returns of the same year so after accumulating lots of statistical data they are likely better able to make credit extension decisions (which also means extending more credit than others) than their banking competitors that depend on CRA data alone.