My response may be more blunt than just reminding them it is an invasive procedure. Something more along the lines of: "Well since you (insert basher name here) are so used to getting things shoved up your rectum, then it may not seem like a big deal to receive a placebo. But I'm guessing the patients find that very undesirable without receiving a significant benefit to treating their disease."

Everyone here is free to use a version of that rebuttal as necessary.

Cheers!