Is it normal to be annoyed when people use "can" for "must"
This generally doesn't happen now that I'm an adult, but I remember as a kid many adults including my parents would say "you can" to mean "you must" or "you have to". Why? Why not speak clearly, especially to children who are still developing the ability to understand when people aren't being literal? Can=/=Must, so if you are trying to give a command use "must". You will only confuse the child and once the child realizes what you really mean you will just come off as condescending as though you expect it to feel better just because you used the word "can".
"Can" gives permission(OK technically that word is "may" but it's used so often I have no problem with that one) or describes ability. Using "can" to give commands is ridiculous when there are plenty of other words to use that would actually be correct. As a parent I will say "now you must..." never "you can" unless I am just giving permission rather than commanding.