Here is the relevent section from the Ashes Cricket 2009 EULA -
YOU SHALL NOT:
* Copy the Program.
* Sell, rent, lease, license, distribute or otherwise transfer or make available to any other person the Program, in whole or in part, or use the Program or any part thereof in any commercial context, including but not limited to using in a service bureau, ?cyber cafe?, computer gaming center or any other commercial location in which multiple users may access the Program. Codemasters may offer a separate Site License Agreement to permit you to make the Program available for commercial use; see the contact information below.
* Reverse engineer, derive source code, modify, decompile, disassemble, or create derivative works of the Program, in whole or in part.
* Remove, disable or circumvent any proprietary notices or labels contained on or within the Program.
The third point is the one relevant to game modifications - though it is a matter of interpretation as to whether changing the graphics of a game or something is considered a modification to the program, or whether that just covers the functional code.
The second thing that would make a difference is whether you are modifying the game through it's own functionality. In theory with some of the other games that scan the program files directory as part of startup (which is a valid testing functionality for the developers), you are not modifying the game code, rather the game is executing external code.
So by that basis, the way that Ashes Cricket was edited on PC would probably fall on the wrong side of a strict interpretation of the EULA, while something like Cricket 07, where you added files that the game just happens to open, wouldn't be.
Of course you can further interpret that you would need to do that modification without deriving anything or reverse engineering anything from the game - which is very unlikely.
That said, most cases have found broader rights to reverse engineer, however the enforceability of an EULA in overriding that right has had mixed results. Regardless though, it's a grey area that most patching operates in.
All that said, you wouldn't have the right to use the logos, trademarks, etc, that form the basis of most patches - however for personal, non-commercial usage, you could probably argue fair use.
For publishers, they could expose themselves to liability - if a company considers that they have material loss because their IP can be put in without the publisher having paid a license for it they could probably sue.
But for that it's balanced off in a cost benefit sense to the PR that might result in - it's not that companies encourage it, they may not see the benefit of people modding games and putting their logos in it, but they may see a negative if they tried to stop it.
So basically, it's a matter of a few legal grey areas combined with no real significant losses from it in most cases. Piracy is far more of an issue - so buy your games and support the developers, otherwise you'll have no game to edit even if you could.