Most likely one of two ways:

1. His employment contract specifies what reasons may justify a "termination for cause", and this wasn't listed.

(1a. eBay didn't want to have to prove that the conduct here met the definition of a specified "for cause" cause.)

2. eBay could have terminated him for cause, but intentionally terminated him for no cause instead in hopes of avoiding negative publicity.

They chose to pay his severance instead of disclosing they had an unethical CEO for 5 years.

Imagine if eBay said why the CEO was fired and then he turns around and starts telling the world where eBay’s bodies are buried.

It’s less risky for the company to do it this way.