People have the mentality that they want to be the big hero and carry of the game. Partly this happens because of the nature of announcements, it is really cool to get double, triple, quadra or even pentakill announced for yourself, meanwhile the people who enabled the carry to do these don't get recognised.
Marksmen as a class are complete glass cannons. They build almost nothing but damage and therefore tend to die immediately if getting caught by basically every other class in the game (except enchanters). They need protection, unlike other carry type champions like juggernauts, divers and oftenly mages, too. These usually have build in survivability to defend themselves.
Personally, I don't see the problem with protecting the ADC. I play all roles, I enjoy marksmen, but I also enjoy to play tanks, juggernauts, and others.
People just have to accept that you can actually carry in different ways, you don't have to be the guy who gets the pentakill to feel rewarded. As a tank, if I properly protect my carries or enable them to get multiple kills with a well executed engage, then I feel that I did a great job.
Or see it that way: Yes, ADCs need protection, since they are huge damage threats for the enemy but die quickly. They become a key target because of that. So if I play a mage or assassin and manage to kill the enemy ADC in the middle of their team, then I feel that I did great because I managed to take out their key member and enable my team to win the fight that way.
People who complain about having to protect the ADC mostly have a small problem with their ego in my opinion. They have to get over themselves. Protecting your teammates is just as honorable as killing the opponent.