I don't think that it is a general issue. More likely this was a very "special" family.

I do sometimes order customized dishes. I adore Spaghetti aglio e olio. Unfortunately, many Italian restaurant only have the original (Spaghetti alle vongole). I don't like clams and I don't eat them. Logically, I end up asking "Can I have the Spaghetti alle vongole but without the vongole?"

I don't mind if they say no. There are lots of alternatives to choose from. But I've never had a restaurant refuse me to prepare me my customized spaghettis.

Another place, where F&F usually wants customized dishes are Japanese restaurant. Some F&F don't like Tuna. The restaurant however offers mixed sushi platters. A simple "Can I have salmon only?" was always accepted.

I've never tried to force a customized offer and always accepted if a restaurant didn't want to take my order.