Hey Roland,
Totally fair question to ask. The Exchange and SQL agents don't work the same way at all so you aren't the first one confused by it (and the documentation could be much better).
If you are after a full backup of the data but not truncating the sql logs. In Backup Exec SQL Agent terms, this is considered a "FULL" backup. Unlike the Exchange agent, a full backup for SQL does not truncate the logs. If you wanted to truncate the logs, you have to run a SQL agent Transaction log backup which backs up and truncates just the logs but not the database.
In the Exchange agent, a "FULL" DOES truncate the logs.
Hope that helps,
Evan