It's not just airlines that employ this tactic. Time was I used to be able to buy tickets online to watch my favourite football team and you could choose any spare seat you wanted.

Nowadays you don't get that choice. If, for example, one person wishes to buy a single seat he can't plonk himself in the middle of, say, five empty seats; he is allocated a seat at one end or the other that group of seats, or even allocated a totally different spare single seat altogether - thereby maximising the opportunity for groups of people to sit together. Seems fair enough.