And those are built off of the original SCI interpreter and work in the same way as it, still illegal. Regarless of how you look at it, selling games that use SCI is illegal.
It's like me stealing somebody's product, making my own based on it and works with it's data, then GPL'ing it. I didn't own the original product, so it is illegal regardless of the fact that I wrote my own intepreter... because it still uses the original file format as well as general way of working.
Copyright doesn't work that way. Copyright protects the actual content, not ideas or looks'n'feels.
Sierra has copyrights on their own interpreter, and FreeSCI guys do have copyright to their interpreter. FreeSCI guys are licensing their interpreter under GPL, so anybody can take it and distrubute it under the GPL terms. If there happens to be a commercial game (ie. non-GPL'd) bundled with the FreeSCI engine, there might be a copyright violation towards the FreeSCI developers, but it certainly isn't sure that they'd win the case, as the developers of the commercial game would most likely ship the FreeSCI engine right under the GPL terms.
About patents, it would be interesting to hear what patents Sierra would have on their algorithms and such. First of all. patents are always geographically constrained. For example, in Europe you usually can't patent algorithms. Then, if Sierra really had some patents on the SCI technology, they'd most likely be already, or really soon expired.
Brian however has a point about the US law system: it doesn't care whether or not Sierra had premises to win the case, they can still outnumber most of us in the number of lawyers.
Well, this discussion is on a theoretical (and off-topic as well

level, as Sierra doesn't seem to be interested in the almost 20 year old games they have, (and most likely, they are wise in their decision of letting people hack around with the old stuff). And then, the SCI environment is very unlikely to be the environment of creating the next block buster game. As a programming exercise, and a hobby, both the FreeSCI and SCIStudio are most likely really entertaining projects.
-Naksuapina