It's unfortunate, that some folks opinions about any fish species, other than those they like, are as trash fish or invasives and they believe they should be tossed on the bank. So, I believe they may have been wrong in that situation.
I've never been anywhere that the whitefish existed, but from what I have read, they take a fly readily and make good table fare if you should decide to keep a few, and it's legal.
I'll presume that those other folks only wanted to catch trout? If you really got right down to it, Brown trout are not native here in the N. America, and can be invasive. Rainbows too in some waters. I agree with you, there's no good reason for wanton waste of any resource, and especially if they are native. Yet, not everyone agrees with that!
In a recent social media post, another fellow had posted that he had kept some Brown trout to eat, while fly fishing, and others got quite nasty with him about it. His contention was that it was legal, and they are in fact invasive, and they were diminishing the numbers of native Brook Trout where he was fishing. I can't disagree with his reasons, and frankly see no issue with keeping some for the table, but again not everyone agrees.
In my own fishing, I've caught a good many Fallfish, Suckers, chubs and some Carp, and many folks have that same negative opinion about them too. In most cases where I was fishing, they were native species, and generally an indicator that the waters were relatively healthy. They certainly compete with trout or bass that may also be in those waters. I've even eaten Fallfish & Suckers, and even though there are much better fish to eat, I wouldn't turn them down, and would not toss them on the bank.