First off, the reason why I put legitimate in quotes is because most people equate legitimacy with the consumer market. However, this isn't always the case. I could've easily not put legitimate in quotation marks and still have been correct. It lays in the context of legitimacy.
With that said, there are several differences between legitimate fansubbers and bootleggers and that lay within the general ethics that fansubbers have created for themselves. Fansubs have always been a means of getting shows that people wouldn't normally get the chance to see to fans. However, to go along with that purpose, fansubs also have existed to bring awareness of a series and possibly garner enough attention so that it may be brought over to the domestic market. When that series is licensed, a true fansubber will stop because our job is done. As soon as you continue subtitling something that has been licensed you step across the line of being a fansubber into a bootlegger. Fansubbers exist as an entity to further the two purposes I said above and going past that doesn't allow you the right to call yourself a fansubber just like how a person doesn't have the right to call themselves a soldier or a police officer just because they can fire a gun. Just because you subtitle a series and you're a fan does not mean you're a fansubber.
There's a whole slew of things I could go over but I think I've given the gist of it.
Edit: bootlegger is the term I use in this context oppose fansubbers. There probably is more apt terms but that's the word fansubbers have been using for as long as I have been one (5-6 years now).