It's highly unlikely that those other engines are using the "Carmack's reverse" algorithm.
It's very stupid that such a simple concept can be patented:
1) Start at the furthest point in the shadow volume
2) Am I within the line of sight of a shadow casting object?
3a) Yes, then draw a shadow on me and everything in the volume and exit
3b) No, then don't draw a shadow on me and move closer
4) Go to step 2 until the 3a is complete or we reach the shadow casting object
I will patent the fastest algorithm to wash dishes...
I will patent the fastest way to stack rocks...
Pretty soon everyone will be forced to work inefficiently so they don't violate all these "valuable" patents.
You probably lived under a rock the last few years, didn't you?
The entire software world is like that for years! Just look at Apple patenting the design of the ipad (a rounded rectangle with glossy surface), Microsoft extorting money from every Android device for trivial patents (the list was published a few days ago and contained such gems like "show the user a 'still loading' message inside the browser window) (and I bet MS gets more money per device than say the manufacturer of the display, or the battery), the trivial patents about the video encodings, the DVD and blueyray spec (yes, they came up with a spec on "how to store data on an external storage medium", and are now collection licenses from EVERY FRICKING PRESSED DISC, even tho all they did was draw up a list on "how to store the author, where to put the files etc.". you can read the stories how MS desperatedly wanted to be in the consortium, by adding their video codec to the list of "official codecs". Becaue everyone on that list gets royalities, no matter what codec is actually used...), the MP3 patent which essentially patents "lets throw away data the user isn't going to miss, anyway) (this time putting Germany to shame), and the list goes on and on and on.
Most patents these days are "XYZ ... on a mobile device!" whereas 3 years ago the same patents where "XYZ ... on the internet!". And before that it was "XYZ ... with a computer!". And XYZ isusually some trivial stuff where the patent took a laywer longer to write than an engeenier to come up with the solution when faced with the problem before breakfast...
Almost all software patents fall on the "it is trivial anyway" rule, most of them have prior art, anyway, and in the EU, you can't patent an algorithm (which all software essentially boils down to), anyway. But look at the US. The doubled their number of patents in the last 5 years or so....
Anyway, kudos to Carmarck to work around and from the inside of such a broken system.