It's very silly for the opponent to throw a tantrum about it. It's a minor tennis custom, it shouldn't be a big deal.
That said, there IS a reason why this custom came about, it's a different situation from hitting, say, the back of the line on a lob.
We generally assume, in tennis, that whoever controls the ball better, whoever does what they are trying to do with the ball better, "should" win the point. If you go for a topspin lob and hit a topspin lob that lands right on the line... you hit exactly the shot you were trying to hit, and did it perfectly. If you aim for a down-the-line shot and end up hitting a down-the-line winner that clips the line, well, great shot! Heck, even in other cases there's plausible deniability - if you aim to hit a crosscourt shot but hit the ball late and hit a down-the-line winner, well, the only person who knows that you didn't hit the spot you were aiming for is yourself.
But a net-cord results in weird situation like where you were TRYING to hit, say, a down-the line flat ripper, and instead hit a dropshot. Oops. Nobody can pretend that they really were trying to hit that dropshot. It's clear that you obviously lost control of the ball, it went somewhere you didn't intend it to go, and yet you still won the point.
Hence the custom of "apologizing" for it. It's not a particularly big deal, the most anybody expects is just a quick raise of the hand...
(If you hit the net cord and lose the point due to it, that makes sense and your opponent has nothing to apologize for - you lost control of the ball and therefore lost the point, makes sense.)