This is more than stunning, especially given the age of the stamp, your short visit and TWOV, not visa application. I don't recall reading about a similar experience anywhere before.

Note that sending people back from the airport involves a lot of paperwork and people. This could not have been just the result of an over-eager new guy.

(And with a visa, I was once asked about a Turkish stamp, but let through after I explained it was just a tourist trip, and no problem at all on many other entries.)

Note that the Turkish stamp is small, and if you have a well-used passport they have to make some attempt to find it.

I'd mention this to your embassy; they should add a traveller's warning about it.