Youtube to MP3

Hello the community. Someone with some degree of knowledge in laws, can teach me why a video made public in the internet, particularly in youtube, can't (Or mustn't) be download in MP3 or MP4 format to listen or watch it offline? Sincerely I don't understand why. Note I am Electronic Engineer, not a person skilled in laws or legal issues. In any case, please, suggest me any way to do it if it isn't illegal.

Many thanks in advance.
 
Thank you Mooly, but I prefer online converters, because this machine has very restricted bandwidth and any video you can watch or listen gets cut excessively, turning it unsupportable. This is why I prefer to download and watch and listen offline. At home, no one kind of internet.
 
... Someone with some degree of knowledge in laws, can teach me why a video made public in the internet, particularly in youtube, can't (Or mustn't) be download in MP3 or MP4 format to listen or watch it offline?...

Isn't the original Q about the legality of a download? I'd like to hear about it. It is/was kind of OK to tape a radio/TV show, but I've never seen any definitive answer to it.