Though a native English-speaker, I've always had trouble understanding bracketed text. It's fairly clear that parenthesis indicate "optional" text, as in below:

The (unusually quick) man stormed into the store and grabbed every lighter from the stand near the cash register.

Above, it's not necessary to include the text "unusually quick," as it can be determined somewhat from the context, but its inclusion indicates a reaction of the author or a point desired to be emphasized. The general idea seems to be that a sentence with parenthesized text should be able to be read without the parenthesized text. While the parethesized text adds to the sentence details, it is not necessary in and of itself.

However, square brackets seem to indicate complete literary freedom to rewrite a quotation entirely:

...[he] had [indicated that he had] made the gesture without [complete knowledge and] familiarity with [Taiwanese] custom.

What are the rules with using square brackets and their general implication? I, as the reader, am completely at the mercy of the author in trusting him that he has conveyed the quote in its proper context and intention.