It's not that they don't count, but rather that employers probably shouldn't be on the hook for benefits received while the employee wasn't actually an employee. If that employee is working two part-time jobs, for example, then both employers should be contributing.
Same for switching between seasonal jobs; if I'm working for Sunshine Ranch as a ranchhand April through September, and I'm working for Powder Bowl Ski Lodge as a custodian October through March, those two companies would need to be taxed proportionately.
Not sure why part time and seasonal workers don't count since that's often the only way to even start working at Amazon.