It has been raised here a few times in the past that Sixt did not include winter tires by default but was asking for an extra fee. This was considered to be extra-sneaky as German law mandates winter tires if the circumstances make those necessary.

In the end it came to a gamble whether one pays the extra fee and is guaranteed a car with winter tires or if one hopes that the car one gets already has winter tires.

Sixt has now announced that winter tires will be included in the rental fees during winter season. Link in German is at: http://www.sixtblog.de/pressemeldung...all-inclusive/.
The blog post talks about reduced rental fees if the weather conditions allow the rental of a car with summer tires. I guess this means that actual rentals will incur higher rental fees. But nevertheless, I guess this is an improvement. Even if it is not really "for free".