"It seems illogical to me that one spouse's Medicare deducted from their own Social Security should be reportable by the other, self-employed, spouse."

There have been many change I chalk up to attempts at parity. Once the ACA happened, many other things were modified to try to provide an equitable playing field. The government requires the over-65 to be on Medicare, and that is always independent and in the name of the individual. Yet, the business expense and tax deduction provisions want to provide equitable consideration to all, whether you are on the Marketplace, covered "at work" and whether the plan is Individual, Family, etc.

The whole "Deducted from their own Social Security" simply means: the government is doing the banking for me, and taking the payment. It isn't really post-tax or pre-tax, because now you venture into how and why that specific SS gross is or is not taxable as a mutliple-dependency analysis. That's too deep for me.