From the definition for “since” in the New Oxford American Dictionary, 2nd edition (on my iBook):
When using since as a causal conjunction to mean “because” or “given that,” be aware that in some contexts or constructions the word may be construed as referring to time.
Here’s an example of when using “since” can be confusing: “Since Britney Spears married Kevin Federline, her career has been a disaster.” While there’s no doubt about the meaning—or truth—of the second part of the sentence, the first part is unclear. Did Britney’s marriage cause the downward spiral of her career? Or did some other events occur after her nuptials that brought about her tragic fall?
Here’s another example: “Since the United States entered Iraq, the country has fallen into chaos.” Did the United States’ presence cause the chaos? Or have other events unfolded after U.S. troops arrived in Iraq that led to the chaos? Here using “since” can cause readers to have opposite interpretations of the same sentence—and can lead to radically different courses of action.
While it’s not wrong to use “since” to mean “because,” it is wrong to confuse your audience. Don’t use “since” to note causation, but instead rely on other conjunctions, such as “as” or “because.”