The classes are for people who only speak German. People emigrating from Turkey likely speak Turkish.
It's about understanding other cultures, and the ones most relevent. Just like how in England, along with learning about christianity, we learnt about Islam.
It's like in the US, Spanish is the most popular second language taught in schools, since you're very close to large numbers of spanish speakers, and have a high number of spanish-speaking migrants. In the UK, amongst a load of other reasons, until very recently French was the most poopular second language in schools, since France is pretty close to us.
> Germans don't have to learn about Turks
Why not? If they reside in the same country, mutual culteral understanding can be key for integration.
> Turks have to learn about Germans and blend in
We can have both, Germans learn a bit of Turkish, Turks learn German, where's the problem?
> If they can't, that's a very serious failure.
Where above is it indicated that Turks are failing to learn German?
Also I can imagine that adding a new language choice is very expensive.