I am not sure what exactly it's expecting. If you remove this user the check passes, but the server fails.
How can it be resolved so the check passes, yet it has the limited permissions MBSA is looking for?
The general consensus I have found is that the MBSA security settings for SQL Server are out of date and might need to be ignored. Most of the security settings are set by default when SQL Server is installed, and as you have found, if you make MBSA happy, SQL Server fails. You may want to consider unchecking "Check for SQL administrative vulnerabilities"
Edit: Security practices
sp_Blitz is a neat little tool, and www.brentozar.com has a lot of handy SQL Server tips and tricks