Perhaps they believe that boys as future men should be treated differently. Maybe they think that boys need a tougher upbringing.
That is outrageously sexist. I don't know how these people sleep at night.
This kind of attitude comes from a time when virtually all countries mandated military service of their young men. This became ingrained in many countries ever since the Great War of 1914-1918 when millions of men from nations all over the world were drafted into vast armies. This unchallenged notion became even more prevalent during WWII, when nations like the US had some 16 million young men under arms in their militaries.
After WWII with the rivalries between blocs of nations, yet another world war was expected and so nations prepared for this coming conflict by doing their utmost to have boys grow up to be strong, healthy, resilient and tough, this for later service in the armed forces of their respective nations.
This is how I for instance was brought up, to expect to fight in a WWIII, this is how the general attitude was of virtually all Americans and other (European) nations. It was simply the prevailing expectation of my younger years, the 1950s-60s-70s. And if there wasn't a WWIII going on yet, there were always proxy conflicts going on around the world that would require a military to intervene.
There is actually nothing shocking about such an attitude. Throughout history men have fought the wars or done hard backbreaking work. They needed to be tough and strong.
It is our present day and age that is something of an exception. And even so, if you follow world news and expectations of the future, surely you can see so many signs of the pendulum swinging back to the notions of the past.