There is a reason why this is so notable. Same with Belichick resigning the Jets job. It doesn't happen that often. These jobs are coveted.
Should people have been aware it could happen? OK. But it almost never happens. You get conditioned to expect the handshake to hold and the timing of other signings wasn't unusual. There have been times when a certain assistant coach signs before a head coach and it becomes a confirmation that a certain head coach will be on his way shortly.
I do think Kraft deliberately tried to screw them over. I bet he delighted in that. It may not have been the original impetus, but I bet it made it more fun.
As it happens, I think the Colts got a better coach. I didn't trust McDaniels before this.