Suppose someone flew to Paris (France = EU), and then using a railpass (say) went to lots of other EU countries, and then returned to the US via Paris. Since crossing between EU countries (in most instances) is not subject PP controls, if one left out some or all of those other countries, how could CBP know? [Back in the 60's and 70's when I traveled to Europe, I did the routing above (long before EU), and even then, stamping passports (except for your air arrival and departure points) was a rarity--from my recollection Denmark was the only place that stamped pps when you arrived by train (and boat).]
In this case, you would be required to list the countries visited on your blue customs form when re-entering the USA. If this information is entered into the computer, it would be associated with your passport number and could be retrieved.