Why couldn't they do it and publish "warning"?

I totally agree.

Such a change is not easy to accept, when you don't inform your customers that they should back up their keys in safe place...

Something that caught my attention when I opened my account in Steemit, was the number of times that steemit warned me that "I could not recover my primary key".
They told me so many times. Everywhere the same warning appeared. Then I made about 15 backups everywhere ... LOL!