CM,
I'm sure USMC IOC was indeed just as important last year, however RIAT/F'boro weren't going to be occurring in the same month back then, unlike this year. That's the biggest difference IMHO.
For what it's worth, I also don't consider the decision related to a wrong perception of public mood; rather just a simple case of where the focus of effort should be. The USMC were a huge part of the joint effort to bring F-35B to the UK and supported it wholeheartedly. That sentiment hasn't changed but current priorities within the Program don't make a UK visit feasible this year. If people judge that a mistake, that's absolutely fine. If people judge it as a pragmatic decision to prioritise manpower, money and effort to make IOC, that's fine too.
Not saying anyone's wrong, just stating the facts as I believe them to be.