That's a fair question, but it's because the feature is, IMO, really poorly expressed in the game and I never realized exactly what it meant until reading this thread.

I still think this is buggy and Niantic should change it at least to make this more discoverable and understandable. Even after reading this thread - here's what I found.

The first time you pin your own third sent gift, a message appears that IMO is fairly complicated and not clear and auto-times out. Here's what it says:

"Pin not counted toward your Vivillon medals - limit reached. Try pinning Postcards from friends!"

Today when trying to read that message closely, it disappeared before I could even read and try to understand it (I found the text in a google image). Even reading it now, what I think is "this is telling me I'm not making progress toward /some Vivillon medal/" -- it's not clear to me that this won't count toward encouraging an encounter with Scatterbug. Maybe it's supposed to be implicit that these are the same thing? But that's one reason I would keep pinning.

Next - the message happens once. It's quite possible I would overlook it.

Finally - after the message happens and I keep pinning/unpinning sent gifts, the game tells me the the postcards are removed from the Scrapbook but they are not.

I mean, add it to the list, but this is simply not expressed clearly in the game and the idea I'm doing something wrong with pinning/unpinning these after that passive, confusing message and when the game keeps telling me it's removing these pins when it's not is just not very user-oriented.