There are different degrees to preservatives. For the most part its just a high sodium factor. Sodium in high amounts can be harmful to your health especially as you get older.
As far as fat loss and muscle building is concerned and why to shy away there are really two main reasons.
1-If trying to achieve fat loss or the appearance of fat loss high sodium amounts increase water retention. If you intake proper water and a low sodium diet then you will shed more of that water weight then your body needs. For some people this can be a pound, for others it can be alot higher. However technically though you may lose weight, it does not affect fat loss.
2-Most products that have high amounts of preservatives also have other bad items in them like refined and over process sugars, bad fats and artificial additives. These things more so than the preservatives can certainly stall fat loss and muscle building efforts.
So as a rule the closer it is from walking or coming to the ground, the better.