I'm on the fence on this one.
Clearly they have changed their policy. In the past I have booked seats together on some flights, others I've not bothered and still got seats together checking in online when it opens.
To be fair if you get 12B and 31E you've got a seat, that's what you paid for, a seat and that's exactly what it is. If you pay extra to pre book then you also get what you paid for. You can't really grumble. What is clearly wrong is that they aren't being honest, they clearly have tweaked the system to generate more revenue. If I do use them in future I will now always pre book. So you could say they've achieved their goal by doing this.
However if I'm paying £50 all in to jet from one side of Europe to the other I can't really moan about that either.