Such shallow reasoning.   There may have been secession even without slavery as an issue.  There were many issues occurring.  To preserve slavery all the southern states needed to do was remain IN the union and support the Lincoln backed Corwin admendment  (original 13th admendment ).
Modern revisionist history ignores what was written at the time and soon after the war.  There war itself did have an underlying cause,  money and power.   When asked why not let the south go Lincoln replied where shall I get my revenue.  The south provided some 80% off federal revenue in 1861 and Lincoln would not let it go. 
If you condider freedom self determination and the United States constitution positive ideas then one has to also consider the Confederate battle flag a positive symbol as it embodies all three.