That's at most 1/10th the cost of the average Samsung phone.

That's cheap. If you think that a safe first-party replacement battery will sell for less than the 79€ that the whole replacement effort takes, then you're fooling yourself.

I strongly suspect that there's also not good language for blocking against third-party batteries (and the phone manufacturers would have good reason to do so because it might result in overheating or worse with really bad third-party batteries).

The people for whom €79 is not cheap are not getting flagship Samsungs, but some low tier $100-300 Android.

Here's a replacement battery for last year's S25 Ultra: https://www.mobilesentrix.ca/replacement-battery-compatible-.... Retails for 14 CAD or approx 9 EUR (11 EUR with a 20% VAT). So yes, 79 EUR would be extremely expensive.