Comment by account42

Depending on the airport and terminal (e.g. shitholes like Frankfurt, especially terminal 2), filling it on the other side might mean a washbasin in a stinky toilet because they'd rather you buy overpriced bottled water. And many airports that do have at least water fountains only have some that seem deliberately designed to prevent you from using them to fill any reasonably sized bottle.

Also, don't count on security not throwing away your empty water bottle anyway just because they can.