Would not recommend fishing while bear viewing. It eats up a ton of time and salmon fishing in the lakes aren't very productive.
If you're going bear viewing, go bear viewing.
I actually spoke to 2 families that did this exact trip last summer as a fishing/bear viewing combo and said they had a blast. They saw tons of bears and caught a bunch of salmon.
Having said that, you do not have to do any salmon fishing at all with them. You can tell them you only want to do bear viewing and they're perfectly okay with that. I spoke to them about it and they said so specifically.
Btw, I was also told by a park ranger that works in Katmai that August is a slow time for bears at Brooks Falls and you might have bad luck and not even see one bear there. Not for sure but possible. They said that Lake Clark National Park is the better place to go in August. You can go on Alaska.org and view a number of different options of bear viewing tours to Lake Clark..