Is "since" a synonym of "because"? [duplicate]

Possible Duplicate:
When are “because”, “since”,“for” and “as” interchangeable?

A few years ago, I was told that "since" should only be used to designate a time period. For example:

Since 2 o'clock, I've been waiting for you.

However, since creeps into the place of "because" quite often.

Since I don't have the time, I won't be joining you.

This second example sounds slightly slangy to me now that I've been avoiding using "since" in this way for the past few years, but is "since" really a synonym of "because"?

Since is used with three different meanings.

• In phrases like "since two c'clock," "since the war," it refers to the period of time between the event and now.
• In sentences like "the settlement had vanished long since." it means ago.
• In other sentences, it means "because," "for the reason that."

The NOAD reports a note about the usage of since.

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. For example, in the sentence "Since Mrs. Jefferson moved to Baltimore in the 1990s, she was not aware of the underlying complexities." it is not clear, especially at the beginning, whether since means "because" or "from the time when." It is often better to simply say because, if that is the intended meaning.

The double meaning of since as after and because is a well known case.

It is for instance cited by famous linguist John McWorther in "The power of Babel".

Here is the complete excerpt:

A thousand years ago, in the language called English, since was a compound word siththan [siððan] from the words from after and that [sið and ðan]¹ and was only used in the chronological "after that" sense of
  - She has been sad since the day her fish died.
The because usage
  - He has to have been there since they found his umbrella in the basement
would only become established five hundred years later.

Please note that the same thing happens today with after in the news when the news media tries to avoid being liable for making unfounded accusations.

The man was arrested by the police after the purse of the victim was found in his car