Not necessarily true; that totally depends on the licence.
You don't have to agree to the license. At that point any legal weight they have is due to copyright. As long as you aren't distributing a derivative work you're fine.
And what grows out of the ground is mine to modify and decompile provided I don't distribute the derivative.. oh wait.
The license can only provide you rights in addition to what the copyright law already grants you.
In some countries making copies and modifications for personal use is completely legal.
If you are doing it for your own use and don't tell anyone the chances you get caught are pretty slim