I built my tube amp without the shield. I actually told somebody that it was OK if he wanted to touch the tubes while the amp was on, it's just that parts of the glass were about 150C.
Small tubes do not get hot enough to burn a hand when touched, unless they are used at the maximum plate dissipation the ratings allow. Power tubes do get quite hot, especially in class A amplifiers.
Alas, very noisy people love the idea of an unregulated world. They think everyone's life and health is disposable, especially the stupid. They think that they are so smart as to avoid all danger. They don't realize that young children are incredibly ignorant (and will hopefully be able to grow out of it someday). They don't realize that there are ordinary lapses in attention or awareness that, rather than causing a permanent disability, could simply be met with an "oops" if there were some protection in place.
There is a need for some common sense, especially if the danger is not all that great. Let's say somebody does touch the 150C tube or pick up a soldering iron on the wrong end. At most, they will get some small burn (unlike spilling hot oil on your hand, touching something hot usually invokes the reflex of taking the hand away and that stops the damage), it may be painful, but not lethal or permanent. I however, will insulate live mains wires so the cannot be touched even if I am connecting them temporarily (for a few minutes) or I will use an insulating transformer because touching a live wire can kill.
Incandescent lightbulbs get hot too, and table lamps usually do not have protection from touch. How did the human race not go extinct because it was possible to touch a hot lightbulb?
We should not have instructions that tell not to put a cat in a microwave oven (or a regular over for that matter) and we should not reward the people that do it even though the instructions did not forbid it. Some things should be understood by all and parents should teach those things to their children. If they don't do that, then maybe those genes should have a reduced chance of reproducing...
Lets say you have a sauna. You can pour water on the hot stones to increase the humidity. Do you really need an instruction to tell you that 1) stones are hot, do not touch them and 2) do not pour gasoline on them?