Writing Rules - A Paradox

There's some writing rules out there that perplex me. Maybe someone can help me understand. Many of these I've seen posted online within the last month. Others are just grammar rules.

Sample rules:

Don't use incomplete sentences.

Don't use adjectives that end in "ly."

Show don't tell.

Don't end a sentence with a preposition. (Although the Chicago Manual of Style says that's a superstition. Odd word choice!)

Eyes that drop or roll.

Don't use a passive voice.

And it's not the rules themselves that perplex me - it's the fact that I can take any book off my shelf and within the first few pages, find that several of those rules have been broken! So what gives? Are these issues important to some editors but not others? Is it that writers and new authors should strive not to break these rules until they are established and then they can write all the bad grammar their heart desires? Or is it something else altogether? Anyone help me here?

Or is this just one of life's great mysteries?