Because, as, since.

Many of my editing classes were taught by professional editors, but they came from a variety of backgrounds. One was a freelancer who specialized in dissertations, another worked editing instruction manuals. After teaching the rules--as laid out by the Chicago Manual of Style--they still had their own preferences for certain sentence structures, diction, etc. They readily acknowledged that they were biased and warned against thinking that other ways were wrong.

Somewhere along the road of my editing instruction, I was under the impression that because indicated causality, since indicated x happened and y happened because of x, and as was a weak causal word or indicated simultaneous events. I edited accordingly. But lo and behold, in researching for this very blog post, I realized that this rule I was so strictly abiding by was not upheld by Chicago: 

since. This word may relate either to time {since last winter} or to causation {since I’m a golfer, I know what “double bogey” means}. Some writers erroneously believe that the word relates exclusively to time. But the causal since was a part of the English language before Chaucer wrote in the fourteenth century, and it is useful as a slightly milder way of expressing causation than because. But where there is any possibility of confusion with the temporal sense, use because. (5.220)

A little earlier, because, as, and since are all accepted as subordinating conjunctions that indicate causality or reason (5.201). So I stand corrected, although I certainly still have my preferences. Here are some sentences that came up recently that rubbed me the wrong way.

Margaret had been thrilled by these books since they all seemed so new compared to everything the library had.

This sentence initially prompted me to doubt the "rule" I thought I had learned. I still don't like since there.

He has forbidden me to go inside since Henry started working there.

Ambiguous. It could mean she was forbidden when he started working there or for the reason that he is is working there. If the latter, then only because is appropriate.

Soon there was no point in washing anymore as she’d just be rearranging the dirt.

All the noisy rancor suddenly died away as no one could see anything and no one wanted to move.

Both of the above could have because instead of as, and that's how I would prefer it, but to each his own. 

Over and out.