Why the ww1 planes aren't covered by the premium planes once a day without a subscription?!

can you be more specific? which plane is not covered?
I think you are getting confused with Mixed wars and WW1. WW1 airplanes cannot be used in Mixed Wars after certain rank. it has nothing to do with the subscription. If you want to continue playing WW1 then you need to subscribe and select that option.

I dont understand why the benefit can not be given to the ww1 period. Planes like Camel and Siemens or even the whole ww1 planes.