If you don't want the beta to correct your grammar, then discuss that with them beforehand.
When somebody hands me a manuscript and says "go," I'm going to comment on everything I see. Including commas. Including spelling. Because it might be helpful, I think, in my stupid misguided head--I'm thinking, it might be helpful for the author to know that straitjacket is not spelled straightjacket, or that this sort of sentence would sound better with a comma, and besides, I never received any parameters telling me what I should or should not comment on. I'll comment on characters and plot and consistency and anything else I can think of, too, but if I see inappropriate punctuation, and I correct it, it's not because I think I'm better than you. It's because I want to be helpful.
If you don't discuss the sort of critique you want beforehand, then it's really not fair to complain about what kind you get.
I do tell this to my critters and I don't know how you got the idea that I don't. Maybe because I sound so mean and disrespectful? First of all I'm starting to realize this discussion's place is in the thread about beta reader experiences that didn't go as planned. But, then again I'm not the moderator here, so I'll answer your message, since it was addressed to me.
I've warned my beta-readers beforehand. Well maybe not all, but after many failed attempts, I understood what i want from my betas. I'm don't prefer line-by-lines because I think they're a waste of time from their parts, as I usually delete scenes or completely rewrite them. The same goes for grammar. If there are huge errors in the text they can go ahead to correct them, but that's not what I want from my betas and I let them know at the start of our relationship. I always apologize beforehand for my English and my lack of a vast vocabulary. I've earned some really great writing-buddies and especially one amazing beta that is completely in sync with me. In fact, before I got myself into this conversation, I was absolutely content with all the people critting my work.
Some are just more comfortable with this style (line-by-line and grammar critiquing, rather than concentrating on the plot) and I understand them. And I understand where wanting to be helpful comes in. But I can tell when they concentrate on the grammar because they essentially have nothing to say about the story.
Ugh, it seems I'm really not good at explaining what actually bothers me about some beta-readers, but just look at the first post I quoted and you'll understand. That person expressed themselves better and I found my experiences mirrored in their thoughts.
Let us not ignore the fact that some correct grammar, when they actually have a really bad knowledge of grammar. And even I, a non-English native English speaker can see it. See the grammar and syntax forum in AW. A writer just recently posted a thread cause she was confused by her beta's grammatical corrections. Turns out the beta was the one who didn't know enough grammar.
I myself had a manuscript beta-read and that beta told me I will never get published because my vocabulary was not good enough, even though I warned her that was exactly what she was getting herself into. Not only did her words hurt a lot, but they weren't at all helpful, unless you consider wallowing in self-pity helpful.
It just doesn't seem fair that writers are accused of getting defensive when we don't enjoy our beta experience. Writer-beta is a bilateral relationship and both parties must feel comfortable with it. It's not just one person's fault if things don't work out.