Punctuation point: comma use in reasoning and consequences

In my last post, I had the following two sentences in succession:

(i) And this is where I grumble about droughts that necessitate four minute showers, because I used to do some of my best thinking in the shower.

(ii) These days epiphanies are fewer because I'm under the water a lot less.

When I'm reading over my blog posts (which, I'm rather ashamed to say I don't always do), I'm reading as an editor and thinking about my grammar and punctuation. So when I got to these two lines, I noticed that I had a comma before "because" in the first, and no comma before "because" in the second. A flag went up in my head because both sentences were similar, yet punctuated differently. Was this correct? Yes, it was.

Both sentences begin with a main clause (which expresses the main idea in the sentence) and are followed by a subordinate clause (which cannot stand alone). The first "because" is introducing a clause that explains a consequence of the action in the first clause. In the second, however, it's introducing a clause that shows the reasoning behind the first clause.

This difference is seen more readily when two main clauses are linked together with the word "so". Here's some examples:

Just before five pm, I went to the post office so I could pay my bills. (The reason why I went -- no comma.)

Just before five pm, I went to the post office, so I didn't get to the bank. (The consequence of my going to the post office; this one takes a comma.)

Here's another example:

Mrs Jones's dog has a muzzle so it can't bite. (Why it's wearing the muzzle, the reason she has muzzled it -- no comma.)

Mrs Jones's dog has a muzzle, so it can't bite. (Ah, now it's not why it's wearing the muzzle, but a consequence of its wearing the muzzle -- that comma changes the reading of the sentence.)

I know I'm anal about such things -- that's the editor in me -- but I have to say I love this kind of nuance I can give just by knowing the rules. Of course many readers won't get the fine distinction, but some will. I can't determine exactly how readers will read my writing, but I can make it as clear as possible so that those who can get it will.