I just hope burger king won't change its mind later on because it's not the first international company to accept BTC only to reverse its decision later.
There is nothing for them to change their mind on.
Takeaway is the platform they are signed up to, and this platform is settling the incoming payments through its partnered payment processors. Burger King itself doesn't know what payment option you are using to pay for your food, neither does it care about that. They want to increase their revenue and for that reason utilize the Takeaway platform.
End result is free marketing for Burger King Germany and free marketing for Bitcoin. Yuuuuu!!