No it is not, it should be yeah, but it's not! It doesn't have anything to do with theft, as long as the software is not open sourced you are not allowed to touch it or modify it in any way
Read the EULA or TOS in any game, you will see this:
LICENSE CONDITIONS
You agree not to:
reverse engineer, decompile, disassemble, display, perform, prepare derivative works based on, or otherwise modify the Software, in whole or in part;
And that's just a tiny part on what you are agreeing on with, again want it or not!