What am I gonna do?

I would find a similar product, with the same amount of protein and calories and sugar or better - it's not hard to find a better choice than Optifast, really - and tell your surgeon you would like to use it instead.  It shouldn't matter if it says meal replacement on the package or not, the important thing is what's in it.  You may need to point out the nutrition stats to your surgeon though because it seems like they often don't take the time to actually read labels.

Personally, if my surgeon insisted I buy a certain brand of shake, I'd be very unhappy.  I think it's unethical.  But if I was unable to find a new surgeon, I'd probably tell him I'd use the BA but that I was going to buy it elsewhere, and then use whatever shake I wanted.