WhatsApp can do that, yes! People are talking about "the line" society would draw, and I expect you just found it!
ISPs should not do that, it violates net neutrality.
ISPs should not do that, it violates net neutrality.
There's no reason WhatsApp shouldn't be treated as a "dumb pipe for messaging" just like an ISP is widely regarded as necessary to be treated as such for data in general.
Where's the difference? Both are companies that want to make profits and face competition. If ISPs were a public state service, then that would be where to draw the line.