If you’re not familiar with SQL Server, the “sysadmin” server role conveys the highest level of authorization available to a login. “db_owner” also conveys a high level of authorization. Both requirements are far more than what is necessary and violate the Principle of Least Privilege. While I strongly disagree with the install-time requirements, I can at least understand the argument: it’s a one-time activity. But elevated permissions at run time are inexcusable.
Most of the time, software companies publish that because they want to avoid the hassle of support calls when people don’t grant privileges correctly. I’ve worked with one third-party vendor in the past who sent me the actual permissions requirements after I pestered them a bit, as I wasn’t going to let just anyone have sysadmin on my servers. But that’s not a scalable approach and does nothing for the next guy who reads the documentation and just gives sysadmin away.