After reading your post and struggling with this on several occasions in the past I did some further reading in BOL and found this in the Checkpoint Topic. I thought this particular piece was rather interesting and applies directly to your scenario. Hope this helps you as much as it did me.
A database is in log truncate mode when both these conditions are TRUE:
The database is using the simple recovery model.
One of these events has occurred after the last BACKUP DATABASE statement referencing the database was executed:
·A BACKUP LOG statement referencing the database is executed with either the NO_LOG or TRUNCATE_ONLY clauses.
·A nonlogged operation is performed in the database, such as a nonlogged bulk copy operation or a nonlogged WRITETEXT statement is executed.
·An ALTER DATABASE statement that adds or deletes a file in the database is executed.
“He is no fool who gives what he cannot keep to gain that which he cannot lose” - Jim Elliot