Linkware license: "You can use these assets for free in your projects, even in commercial works. All you need to do is add a link back to the Streamline Icons (www.streamlinehq.com) website."
I am not a lawyer, but these icons are necessarily public domain works, if we consider they are made out of geometric shapes that are too simple to be copyrighted (https://en.wikipedia.org/wiki/Threshold_of_originality). Claiming copyright on these icons would mean they are derivative works, which would mean OP violated the copyright of the original works.
As long as it's not a photo showing them or the loved ones, or a piece of art they created themselves. 'Meme culture' mostly seems to reject licences because it's a bother, not out of idealism.
That's a perfectly valid reason. Requiring a license for everything is pathological. Unlimited license for anything is also problematic, as you point out, but there should be some principle of balance between moral obligation and administrative overhead.
We already have corporate entities that claim ownership of particular words or colors, not because they invented them but because they laid claim to them and want to extract rents therefrom. Branding serves a rational commercial function, but when taken to extremes it begins a resemble a form of psychic territorial aggression.
Linkware license: "You can use these assets for free in your projects, even in commercial works. All you need to do is add a link back to the Streamline Icons (www.streamlinehq.com) website."