This is why treating SD cards as internal storage is stupid. It also reduces your phones performance to the speed of your card, which isn't as fast as internal storage..
..Very annoying feature of Android...but it is a developer decision to allow an app to be moved to an SD card.
This thread is coming up on top of the google searches regarding adoptive storage and marshmallow, so I'm bumping it for the sole purpose that many other people will be looking to this thread for answers.
I don't personally know the catch-all answer to this question, but using a rooted device with "adb shell sm set-force-adoptable true" function should work for most I would imagine. The question is, is there an answer for those that don't wish to root their device?
And BlurryEyed, stop spreading fear and misinformation regarding this topic. Almost all of your responses are incorrect. Regarding adoptive storage, it is not the developer's decision to allow an app to be moved to the SD card. Treating an SD card as internal storage in situations where the user thinks it's the best option is NOT "stupid".
Yes if a developer is skilled enough he can maneuver his app from landing on an adoptive storage SD card, but 99% of app development tools won't effect this and the other almost all of the remaining 1% of developers wouldn't take the extra time coding to prevent their users from doing so.