The long and short if it is, it's about legal issues. Logos need to often be trademarked or registered. Using free resources often means the designer does not own the rights to the free item. So, rights certainly can't be provided to any client if the designer doesn't already own them. A copyright can not be acquired for something if you don't own the rights to it.
Most certainly don't want a logo that uses free resources which anyone can also use. This completely defeats the purpose of creating a unique identifier for the company. Any competitor can find, and use the same free items and dilute the brand. There's nothing which could be done about that.
So, while yes you could use free resources, it's just bad design or bad business if you do.
As for charging for a logo using free resources, the client may not be getting what they think they have legal rights to and the designer may be liable if they aren't disclosing everything properly. I am not an attorney though. Logo artwork should always be free and clear of any rights claims by others. Just because something is free to use, it does not mean there is no copyright on the item.
It is always best to create your own unique content for logos. Even if the idea or concept is similar to something else, original artwork is often a must for logo.