Yes, it is true that every standard dictionary informs us that "since" may be employed in the sense of "because." I beg you, fergit it!
What the usual suspects do not say is that the usage is slovenly, sloppy, careless, unthinking, and likely to confuse the casual reader. The practice cannot be condoned, even when it is employed by a justice of the U.S. Supreme Court.
This is today's rant: In an opinion in June 2005, Justice Antonin Scalia wrote: "I would hold that, since there is no clear statement of coverage, Title III does not apply ..." He meant " because there is no clear statement ..." In an opinion in a criminal case just a year ago, Scalia wrote of terminology that is misleading "since we hold that in all capital cases ..." Again, he meant, "because we hold ..."
Scalia's colleague, Justice David Souter, is a repeat offender. In Georgia v. Randolph last March, he wrote on page 10, "Since the co-tenant has no recognized authority ..." On page 14 he wrote, "And since the police would then be lawfully in the premises ..." More recently, Souter let us know that, "Since only some orders qualify ..." In each instance he was talking causally, not temporally. He meant BECAUSE!
The stylistic misdemeanor is especially notable when a writer employs "since" in both capacities just an inch or two apart. Thus, in The New Republic last February, we find first a causal "since": "Since it's easy to imagine a sensible way of protecting privacy ..." In the next sentence, we're temporal: "Since Sept. 11 this magazine has argued ..." In the next paragraph we're temporal again: "Since the act was passed, almost all of the libertarian objections have focused ..."
Eight months ago a different writer for The New Republic gave us a double whammy: "Since this is the first time in three decades that Iraq is in the hands of the Iraqis, the present situation is exhilarating, but since there may be no such people as the Iraqis, it is troubling."
Horrid Examples abound:
From The Washington Post's restaurant critic: "Since one size doesn't fit all when it comes to restaurants, here's a cheat sheet ..."
From Preservation magazine, remarking the centennial of the Antiquities Act: "Since there will be 100 candles burning on it, we'd better have a fire extinguisher close by."
From bridge columnist Philip Adler: "Since West needed three hearts for her two-heart raise ... And since West had the top clubs ... South went down one at four spades." (Truth to tell, South went down because she mixed up her spades and clubs.)