In effect, there are actually a few licenses...
First is basic copyright. If I don't put anything into the license, then I still have basic copyright on my artwork (note, exact copyright law will vary, however for Australia this is automatic).
Second, is any additional restrictions (or allowances) I place onto the content in the license tag. You points about finding this 'license' information are valid if I were to just redistribute another person's content without modification. However, as soon as you introduce modification of the content, you are already taking most of those steps. You are just skipping the reading of the license section in the config.txt file (you'll need to open the config somehow, either with notepad/etc or CCP, to change the name...). Many creators put a 'full' license in here, so as to make it clearer.
The license in the 'config.txt' file is available by opening the 'asset details' panel, then clicking on 'view license'. The asset will need to be selected of course.
Third is if you are uploading to the DLS, there's the DLS terms and conditions (aka, the DLS license agreement). Namely, the DLS terms and conditions (which are agreed to when uploading) require you to obtain permission from the original creator (if you are the original creator, then hopefully you've given yourself permission!).
Now I am going to pick your reply apart a little, so as to clarify my answer on it.
If I was a new person how would I know to open CMP
This is outlined quite nicely in the Manual. You'll also need to open CM to be able to redistribute the content in the first place, let alone reskin the content...
find the piece of content
Again, the ability to do this will be necessary to actually redistribute, modify/reskin, or download the asset...
open for edit in explorer
Again, the ability to do this is necessary if you wish to reskin or modify the asset. It's at this point that simply 'redistributing' an asset (without modification) would be an easy mistake for a new user. And is one that is understandable. When it does happen, we explain the situation, remove the item from the DLS, and all parties are happy (we haven't actually banned the user, and they generally will have learned something

).
either open the .txt file named license, or open this "config.txt" file that I don't know what it does so do I really dare open it?
Well, if you are reskinning for the first time, you'll generally be following a tutorial, which will tell you to open the config (either with a text editor or CCP) to rename the asset. Even if you aren't, you'll generally want to rename the asset, and you will normally end up here. The license is quite often near the top or middle of the config, and on many items this will fit onto the screen.
Then If I do scroll down to try and find the license.
This is simply taking some effort. And if following a tutorial, then the tutorial should be explaining that you should head to this.
Or the other option open the asset in this "content creation manager" that I have no idea what it does and do I want to risk it when I just got trainz working?
I would be wondering if anyone who 'just got Trainz working' would actually be attempting reskinning at all... If they are, then they will (again) quite likely either head to CCP or a text editor to open the config file so that they can rename their item.
I do agree, a new user will not always see the license section. However, that doesn't give them permission to do what they like. If you don't see the speed sign, does that give you permission to exceed the speed limit?
However, if/when they upload to the DLS, they do agree to a license (by ticking, or clicking on, a button that says "I have read and agree with..." or similar), which does specifically state that you must have permission from the original author.