I think it's fair to say, regardless of the Public Lands Act, being a dick to a landowner is just asking for an unpleasant day for both of you for virtually no benefit, and will remove access across private land for other activities further. Sure you may be allowed to be on the river, but you might have just burnt a bridge for someone else who has been using the same land for hunting.
I routinely fish on public lands next to private, and make sure to go out of my way to be kind and respectful when i run into a landowner. They may not have a right to kick you out of the 'public' section, but I'd rather have a good relationship with those that i run into, such that if i ever need anything they're more receptive (if you break your arm 12 km up the river, i'd much prefer to cut across land and not get chased off because you've been an ***hole to them in the past). Don't jump fences, dont open gates, be respectful, clean up some garbage be kind, keep your dog under control (and likely on leash) and 99% of your interactions will be positive. If you're going around looking for issues, you should probably take up a less quiet sport..
FYI, last time i asked for access from a landowner, not only did they provide me access, they gave me a key to the gate that allowed me to shave off 3 km of hiking by driving. Landowners aren't inherently enemies..