Why doesn't the Serbian border police let you re-enter Serbia at Belgrade airport and then re-exit?

I recently flew from Belgrade to Stockholm, arriving at Belgrade airport at 2 AM, with a flight leaving at 6 AM.

I had to change Serbian dinars to euros, and the only open bureau de change was located in the baggage claim area.

So I went through departures immigration as usual, and then tried re-entering Serbia through arrivals immigration (accessible from the departures area) with the intention of re-exiting through departures immigration.

However, the only border officer then told me that he could let me re-enter, but wouldn't let me re-exit until my flight had departed. In other words, if I re-entered, they would not let me board my flight.

Fortunately a kind airport worker offered to take my dinars with him through immigration to the bureau de change and exchange them for me, and did so.

However, this makes me wonder: why would Serbian border police do this?