KS2 teachers, please explain to this secondary teacher why...

[Quatrefoil]

... you tell children they are not allowed to start a sentence with 'and', 'but' or 'because'?

Because you are.

And that's a fact.

But I'm willing to be enlightened if I'm wrong.

We teach that it is not grammatically 'correct' as per spag test lists of what conjunctions can be used at start /start & middle of sentences however that authors regularly break the rule and once they understand the rule they can too.

Just like exclamations have to start with What or How!

We have that conversation quite frequently. I often tell mine that there are certain things I need to teach them and ask of them because the government have put rules into place, which they will get tested on. In real life, they might not always be followed by the majority of people. That's why I have to teach them that exclamations (for purposes of the stupid SPaG test) start with 'How...(bloody ridiculous)!' or 'What...(a load of bollocks!)'. They generally know how to use an exclamation mark, but might get penalised in the paper, if they don't start with either of these two words.

I've additionally banned mine from starting sentences with "also", mostly because I find it quite irritating. If it's a personal preference, they know that this is the case...and delight in trying to wind me up with it.

I teach in secondary and support the not beginning a sentence with and or but in primary. By secondary they're able to see how to subvert this, use it effectively, and get why they were taught this in the first place. None of them seem to mind and none are confused.

Exclamations do not always begin with what or how but those which do need to have the exclamation mark at the end. What a beautiful sunset! How incredible!
Why are KS2 teachers making such a fuss about this? It's correct punctuation.

Because I have to teach them rules before I can teachc them how to break the rules for effect. My children who can't string a sentence together don't understand why they can sometimes at csnt at others so until they're ready to move on to this it's easier to fix their writing by saying they can't do it.

Exclamations do not always begin with what or how but those which do need to have the exclamation mark at the end. What a beautiful sunset! How incredible!
Why are KS2 teachers making such a fuss about this? It's correct punctuation.
It's correct punctuation, but it's not exclusive. As you said, they do not always start with only these two words. It's the idea of teaching them not to use exclamation marks unless there's a "what" or "how" at the beginning, which is making things appear rather silly. It's not us making the fuss about it. We have to teach certain things...end of.

I particularly love the training sessions, where you get told that they need to know things for the test,...but they still have to use them differently in their normal writing. Of course, though, we aren't meant to teach to the test,...well,...maybe just a little,....oh what the heck, just drop any creativity in writing and teach nothing but SPaG....but don't make it obvious.