One possible reason for your specific situation...
Not all of these cases were made by the same company, so Omega used several over the years. There are certain instances where the current supply of replacement bezels won't fit on some older cases. If you need a new bezel (or case back) for one of these specific cases that the current replacement parts don't fit, then the only choice is to replace the entire case. Omega usually gives a very reduced price on a new case in these circumstances.
I'm guessing, but the most likely scenario is that your case fell into this problem, and when they explained it something was lost in translation. The people who do the talking often have no technical knowledge or training, so it's easy for them to get confused regarding the specifics on something like this. Easier for them to simply tell you it's part of the case than to explain why they don't make the correct replacement parts any longer.
Look around the forum and you will find lots of photos of bezels removed from Speedmasters.
Cheers, Al