"Pleasure fishing" can just be a term to distinguish it from match fishing - but it doesn't mean there's no pleasure in fishing matches! And some of that pleasure is unrelated to prizes, but connected to the company, the craic, the rivalry, the challenge and so on.
Language can't be taken too literally, and labels often don't stand examination. The pleasure in "pleasure fishing" could be anything from chasing a target, to tackling a difficult water, to mastering a new method, to catching whatever turns up, to just being there.
And does anyone experience only pleasure from fishing? I love it, overall, but I wouldn't pretend it doesn't involve some frustrations, disappointments, dull moments, discomfort, tedious chores.....
It's a good question, whether anyone goes fishing for anything other than pleasure. I'd go so far as to say some people go fishing because they have to - it's a compulsion. On a continental weekend away with a girlfriend, we were strolling through the town by a river. I realised I was appraising the water. This looked like a good roach swim. Bet there's bream in there...... Embarrassing but true.