Winter tyres are not required by law in Germany (I believe it is different in Austria). In Germany the driver is only required to "adjust" his/her car to the "current weather situation" (§ 2 Abs. 3 a StVO). In several parts of Germany you hardly have a real winter and I appreciate that I have a choice when arriving in DUS, CGN or TXL to say "Yes" or "No". Sixt in Austria forces me to "buy" winter tyres during the mandatory period (November 1 to April 15) may there be "winter" (= snow) or not.

Assuming we follow your proposal that all rental cars have to have winter tyres you can bet that the prices will be higher. Of course they "charge extra" and this extra charge can be on the bill as an extra charge or included in the general rates. What is the differnce? As the customer you have to pay for it.