You'll be pleased to know that if you went to the places where they do have the truffles in the ground, you do not need the restaurant to be «expensive» to be served real truffles. Black truffles are sliced on your "normal" ~10€ dishes.
You are misreading it completely: it is a preliminary "Do you slice the truffle on it? How is the dish prepared?". There exist perfectly appropriate ways of "«insisting»" ("Thank you, good, I would like to have it sliced in presence").
You put the slices on top: they are not dishes with preliminary preparation (which imply mixing the ingredients). It is just normal to do it in presence - and definitely sensible.
Waiters in restaurants used to serve the real deal expect you to do this, just like sommeliers will expect you to taste the wine before agreeing to buying the bottle.
They want people to know they're served real truffles, because that's what weeds out competitors cheating on products.
Please don't do this, that would be obnoxious. Treat waiters with some level of respect.