I do think there is a difference between a helmet logo and a sweater logo. A helmet is a definitively secondary piece of equipment. A jersey is the emblematic of the team/sport. Its the hockey equivalent of a football player's helmet or baseball player's cap.
In regards to ads being plastered all over european teams uniforms, I honestly don't think thats a real comparable. They treat uniforms with a lot less reverence in there than in North America, its a different culture. Not only are they far more likely to embrace ads on the uniform (I have British friends who get excited when a cool company/logo graces their team's soccer kits) but the uniforms themselves don't seem as important. You've got teams 100+ year old teams like Chelsea that change it up all the time, as long as its blue it doesn't matter. Its a yearly thing.
Meanwhile the Bruins change their socks from gold to black and a bunch of people get irate about it. Colorado changes their secondary equipment colour and it sparks endless debate. This isn't just a thing for sports logos weirdos like us, either. I've seen Maple Leafs fans argue in circles about whether they should have one hem stripe or two. To a lot of people sticking a logo on the Montreal Canadiens or Chicago Blackhawks sweaters is nothing short of sacrilege. I do believe the NHL will go ahead with it and I do believe its whats in the best interest of their bottom line. And I don't believe it will cause people to stop watching, but I also think they'll see a very large drop in jersey sales if they start sticking ads on them.