It's so bloody rude because the person sending the email assumes you are so unimportant that you sit in a box all day just waiting for him to send you this email so you can leap to your feet like some kind of fucking dog eager to help, because you have no other human relations to deal with today than Mr Fucktard, because [he thinks] his social status is unquestionably higher than yours and saying please and thank you are mere formalities, so the fact that he's said them means you should be even more unceasingly grateful and fawning for his bloody email.

I get people behaving like that to me all day, but more obviously intentionally i.e. to my face in person and to a greater degree. As a single woman with no children, it's practically the whole of my life experience except at work and among goths. It makes me want to shoot them in the face and paint obscenities on their bodies using their splattered brains.

As mentioned, in text form this is conditional on their being a first language English speaker; a non-first-language speaker may just be trying to be as concise as possible so as to risk as few mistakes as possible, while remembering to say please and thank you. Admittedly face to face I've had about as much patronising shit from first as nth-order English language speakers.

The first paragraph of that sounded so unreasonable because I missed the following train of thought:
1) Writing step by step instructions is time consuming and not your job
2) The help files contain step by step instructions anyway
2b) Or you can sometimes just poke things until it does what you want it to
3) They haven't read the help files or poked it because they are SO BUSY and their time is SO IMPORTANT that they didn't bother
4) But they assume yours isn't SO IMPORTANT that you don't have time to WRITE the damn things.