As for Block Party Bash, it's actually a very carefully orchestrated, synchronized and consistent show delivering the same experience to guests thoughout its show stop. It uses classic American party songs because the story of the show is that it's a block party thrown by the Pixar Pals and the Pixar characters. At block parties, they play classic American party songs. Hence the song selections for Block Party, which always manage to get the whole audience in one heckuva mood. The parade is not some mental exercise, it's designed to be as much fun as possible, and with characters ranging from Woody to Boo to Gypsy, acrobats on rolling trampolines and some of the most talented, energetic and attractive dancers around, it sure is fun.
Regardless, this isn't the first time a parade hasn't "fit" with the theme of the Studios. The Toy Story Parade, Mulan Parade, Hercules parade, etc had nothing to do with old Hollywood. And like those parades, Block Party features characters from Disney movies. Which are made at studios. And therefore, it fits.