I noticed this change too. It has benefits and a drawback, I suppose.
I don't know if you have noticed how the solo editor button and edit active part button interact, so to speak.
Building on the OP's example: I've opened two parts in Key Edit.
solo off, e.a.p. off: You can hear all the parts in the project currently playing and can edit all parts in the editor.
solo off, e.a.p. on: You can hear all the parts in the project currently playing and can edit only the active part in the editor.
solo on, eap off: You can hear only the parts open in the editor and can edit all of them.
solo on, eap on:You can hear only the active part open in the editor and can edit only the active part
To get the Cubase 6 behavior now, you have to solo all the tracks you want to hear in the project page, then go into key edit, turn off the solo editor button, and activate Edit Active Part Only.
I find this new behavior better. I agree there should be a pref to get the old behavior. I won't comment on the likelihood of seeing that– I have learned to be flexible, and the new way is now integrated into my 'workflow'.
"We've always been at war with Eastasia"