Why isn't Turbo Tax subtracting my Part B Medicare premiums from my total Medicare earnings?

Where do you expect them to be subtracted?   What are Medicare earnings?  You mean your Social Security SSA-1099?   Medicare part B is only a Schedule A medical deduction if you itemize and don’t take the Standard Deduction.   

The Medicare Part B premiums are entered on Schedule A as an itemized deduction.  They do NOT reduce your Social Security benefits reported to you on the SSA-1099 in box 5.  Look at your Social Security statement.

To enter Social Security benefits reported on form SSA-1099

Click on Federal Taxes (Personal using Home and Business)
Click on Wages and Income (Personal Income using Home and Business)
Click on I'll choose what I work on (if shown)
Scroll down to Retirement Plans and Social Security
On Social Security (SSA-1099, RRB-1099), click the start or update button

Or enter ssa-1099 in the Search box located in the upper right of the program screen. Click on Jump to ssa-1099