I think that's actually hard... because you have to know what the text is you're redacting to get the spacing for the letters right. You want to redact specific words, but keep the rest of the document the same. This means that you need to know the letters (and their order due to kerning).
The best way to redact a PDF is with a printer, a sharpie marker, and a scanner.
No, it's entirely straight forward to simply remove any curves and glyphs that intersect with a region, even if all text was converted to paths; any vector graphics editor can do it.
It's definitely not as simple as you're suggesting. PDF viewers are not vector graphics editors so they'd have to implement the whole intersection algorithms, and even though PDFs don't reflow they can still have text so now you need to figure out if you delete "is" from "this is hard" you need to calculate where "hard" is.
The best way to redact a PDF is with a printer, a sharpie marker, and a scanner.