Good point!
Let's do both tings, but I wouldn't compare seating on the subway/metro for 15/20 mins to seating on a plane for 2 or 3 hours with Bubba invading our space on the side and another seat in front of our knees.
I don't manspread if there's someone seated next to me but if the next seat is empty sometimes I do it.
Some folks even put their bag on the seat next to them, I'm sure if the train is getting full they remove it.
Throwing bleach at someone else is not a proper way to behave. I would have a strong guy seat next to them and show them how it feels.