It can be. I suggest that perhaps you need a change of tutor.
1. and your point is your children should aim for straight Es?
1.1 it's weird that my own experience is exactly the reverse, maybe one of our experiences is a fluke. I leave it for others to comment on which it is
1.2 the point of our education system is to score As. This is how the system works. I have a picture of you as a pro-sprinter, who does not believe in trying to run as fast as possible. What is the point of hours of monotonous training for months and years?
2.1 you're just saying "give up before you even try, it's just too hard".
2.2 the reason for tuition is so you can get higher marks. If your kid is an idiot who always brings back Fs, you send him to tuition to raise it to an E or a D. If your kid is already getting Bs, maybe it can be raised to a few As. If he already has a few As, maybe they can all be As.

Tuition is just extra practice. If your kid is learning to play the piano, you don't tell him, "don't spend 1 hour on that stupid piano everyday. Go out and play with your friends". If he's learning to drive, you don't tell him, "why waste money on classes, you don't need any practice. just go sit for the test".

The thing you need is balance. No kid should be in class from day break till well in the night. Unless he really wants it, and the parents can afford it. But if the kid tells dad, "I'm having problems with maths and really need this tuition class", and dad has no money ... well, to bad. He might never be an engineer like he wanted to be, and have to settle for being a technician, but life's like that. Frequently unfair. But if dad has the money, but prefers to spend it on beer ... well, it's his money, but don't expect other people to applaud you for it.