Risk/reward. Most of the kids who don't throw hard don't ever get seen and don't get a shot at the next level. There are exceptions, but they are just that, exceptions.
It was interesting to hear Orel Hershiser say that one of the pitchers in the CWS who was doing very well had little or no shot at going anywhere in pro ball. The pitcher was working in the mid to upper 80s with very good movement on his fastball. His off speed stuff was decent but not spectacular. His fellow announcer was indignant that Orel would say that. Orel explained that pitchers like that sometimes get a chance but very seldom get past A ball. It is a numbers game and the pro's know that the payoff is better going with a lot of kids who throw hard and finding a few who can pitch rather than going with a lot of kids who can pitch and hoping that one or two will be able to pitch even better.
The same happens in college to some degree. A RHP who throws less than 85 isn't going to get a shot unless he's proven to have a lot of pitchability. A far less effective pitcher throwing 90-91 will get a shot.
Of course we shouldn't treat or condition young guys like they are in pro ball. That's why parents need to be involved, to make sure that doesn't happen.
The reality is that there are ways for kids to work on increasing their velocity that are not likely to injure them and some kids can benefit. Now if a kid is throwing 5 to 10 mph harder than any of his peers then it probably isn't worth even a small risk to work on throwing harder and the kid needs to focus on learning how to pitch.
There's nothing wrong with a 10yo playing a bit of long toss as long as it isn't overdone. That is what is good about the interval throwing program. If it hurts you rest a day or two then go back a step in the distance before trying to push it again. At that age it will probably help them learn how to throw harder. At that age they are actually less prone to serious injury than kids who are into their growth spurts.
BTW, you can learn to throw hard and learn how to pitch at the same time. Trevor Bauer is a good example. He built the ability to throw hard but he has also studied pitching and knows how to set up hitters, etc.