In some sense, liqueurs are their own preservative. However, there is still something to be said for using sulphite when appropriate. Sulphite is at the very least a popular sterilization substance. Wine makers routinely wash all their equipment and especially their carboys and bottles with a moderate sulphite solution. As I say elsewhere, it never hurts to have really, really clean materials.
Liqueurs are still susceptible to oxidation even if it takes a long time.
Oxidation can be thought of as the process that turns fruit brown. Oxygen helps break the complex sugars and other neat chemicals down into smaller component parts. Once the skin of a fruit is punctured, oxygen gets in easily and starts breaking the area surrounding the puncture to brown mush. Essentially, the same thing is happening in liqueurs. Over time, and especially with bottles that aren't really air-tight, enough oxygen gets in the bottles to start turning the liqueurs into brown mush.