Every app is controlled by the developer. It is up to them if they want to allow screen rotation or not. Apple does not control this, if you would like to see this feature implemented into the app, get in touch with the app developer and suggest to them that you would like to see this feature added to their App.
I know how frustrating this can be, a lot of the apps that I use for work are not allowed to rotate with the screen and I too like to use a keyboard and work in landscape mode but the apps just do not allow it. Everything will be rotated, then when I go to the app that I use for work, I have to rotate everything back to be able to use it because it does not rotate like everything else. Unfortunately this is something that just has to be lived with unless the app developer decides that they want it to allow screen rotation.