What an unfortunate situation.
My sympathy to those who backed because a single player offline mode was promised. I find it irksome that a few posters are brushing aside your concerns or even mocking you; I wonder how some of those posters would react if their own key/favorite promised feature was altogether cut from the project.
On the other hand, after reading Michael's explanation and taking it at face value I regretfully agree with FD's decision. If it was truly a choice between a dynamic, evolving galaxy and a single player offline mode than FD was going to break a promise no matter what course they chose. More players plan to play in the dynamic galaxy than single player offline (although I acknowledge that the latter group is substantial), the dynamic galaxy model is more innovative and future-oriented, and FD has already expended considerable effort creating the dynamic galaxy. I admit personal bias in this area, as the background simulation is the feature I'm most excited about.
However, slipping this change into the newsletter and greatly understating it (Michael's subsequent explanation made it clear that we will need to be connected constantly, a far cry from the "time to time" mentioned in the newsletter) was the worst possible manner to communicate this change. It certainly didn't help that the community was led to believe that this newsletter would contain substantial information on what features would be in for gamma/launch and that the newsletter was devoid of this type of information. The beta freebies are nice, but make poor substitutes for this kind of information and were poorly timed, sadly causing them to seem more like damage control and sales tactics than a heartfelt thank-you.
What should have been done, and should still be done, is for David to make a separate announcement explaining reasons for the change, apologizing to backers who were counting on the feature, and offering refunds to interested backers. There can still be a lot of positivity to this announcement, and it would be a perfect time to further highlight some features of the dynamic galaxy that this change is enabling. Those backers who are dependent on single player offline to play at all will likely ask for a refund, but I imagine that many players are less upset with the decision itself than the way it was communicated. Letting backers know that the decision was not made lightly, that every alternative was tried (which presumably is what caused the decision not to be made until recently), and that you empathize with their disappointment would go a long way in smoothing this issue over and reassuring the community's trust in the project as a whole.