Since they invented restaurants.
If you want a meal made for you, you go to the business and ask for a meal. If you want someone to lawyer for you, you go to the business and ask for some lawyering.
"I'm hungry"
"Ask your chef to cook you something."
So why do we hear this in law?
I once got asked to sign a stupid contact. I was pretty sure it was not enforceable, and if I didn't sign I'd be fired and make an annoying enemy.
Me and 3 other guys piled in and hired a lawyer for like an hour to have him look it over and tell us if we should sign it or quit.
In the end, my split was $200 or so, and I didn't really learn anything that googling hadn't already told me. I signed it and then quit on my own terms a couple months later.
Not many people these days, even devs, who will just chuck around money like this for far too little marginal benefit over (as someone else in a comment said) "asking randos on the internet".
And if presented with the same scenario again, I'd probably do it again, though probably with some chagrin.
But my point is, $200 isn't a ton but it's not nothing, and if you're already potentially going to be fucked, the probability of $200 being in the "not nothing" category is probably higher. And if it wasn't something a few other people were stuck with too, it'd have been quite a bit more. (Hard to figure the exact bill because one of us decided to talk to him for a lot longer to check on some specific things, so we just venmo'd him for what he thought our % of it was).
So - I guess what I mean is - even for fairly privileged people, "just check with a lawyer" is not something you "just" do.
And I find it super annoying that assholes (like LinkedIn for OP, and the person who was forcing me to sign the contract) can just write shit down and throw it at you and then you're supposed to spend $500+ to check them on their bullshit or you're the one in hot water.
(Though, that said, OP's example isn't perfect because using the LI logo probably should've been a "hmm wait a sec" moment for him. But also if all LI emailed him about was "stop using the logo" then he similarly probably wouldn't be feeling the need to seek legal counsel).
It may not be a system that is geared towards fair play, but if you want to play at all, you really should have a lawyer, too.