I have been following this trend for a few months now. Previously (6+ months ago) there existed a seat block for OWS (non-AY+). Later I started using my OWE (non-AY+) card and noticed the seat block had disappeared.
I always check using ExpertFlyer which differentiates blocked versus occupied. On EF you can still see many seat blocks - just not around my OWE seat
Much like the recent complimentary Wi-Fi change, I believe Finnair intends to encourage Finnair Plus Platinum members, and discourage other Emeralds to fly with them.