When we have a database like sharepoint where we are unable to alter the recovery type to simple we can stop the alter from happening with a registry change
http://www.symantec.com/business/support/index?page=content&id=TECH126063
HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Backup Exec for Windows\Backup Exec\Engine\SQL Server\Number of Non-Log Backups Before Warning
The default value is a decimal value of 10
Increasing this value will increase the number of full database backups it will take to display the exception
Setting the value to 0 will disable this exception from ever occurring. Symantec does not recommend setting this value to 0.