Technically, they should be okay on fair use basis, and not for libel or slander against the person depicted.
Actually, not really. People—you, me, celebrities, whoever—have the right to control the use and distribution of their likeness for both commercial and noncommercial uses. Celebrities tend to have fewer grounds to object to the distribution of their image, since most are doing publicity for various and sundry things now and again, but there are some limits on what you can do.
There are a few exceptions to that general rule—if someone is taking a photograph of a large group of people at an event of some sort, and your face happens to be close enough to be distinguishable, you can’t stop them from releasing it. And when it has to do with actual newsworthy events, your rights of control are fairly limited; you can’t stop news photographers/videographers/artists from capturing and distributing your image if you’re arrested, since that’s a matter of public record, for example. But otherwise, something like a morph/image that’s deliberately aimed at capturing the likeness of a particular person, and then distributed—either for profit or otherwise—is the sort of thing they can go after you for doing. That’s why there are general and specific releases for photographer’s models and other such uses.
Not that I have any idea of the inner workings of the DAZ legal counsel’s office, but, for example, I suspect the amount of legal paper dealing with Reby Sky is truly staggering, since she’s based on a real person who participated in the creation process. Model releases would be the least of it.
The “Mikaela” morph could actually have gotten it from two different directions. Megan Fox and/or her representatives could have objected to the distribution of her likeness without consent—especially since it could possibly have been used for commercial purposes without her consent. (I assume the terms of the morph said that such uses weren’t allowed, but that wouldn’t stop anyone from actually doing that.) And, as noted, since it was using the likeness of Megan Fox and the name of her character from a specific film, the owners of that particular intellectual property could also have objected.
In general, for distributing a freebie, unless they’re feeling particularly vindictive—or unless the RIAA or MPAA are somehow involved, in which case you are terribly terribly screwed—unless they can demonstrate some particular sort of damage linked to that thing, they’ll probably just do a cease-and-desist. If you’re selling the likeness, all bets are off.
Disclaimer: I am not a lawyer, just someone who has had to do a horrifying amount of research into copyright law. Not legal advice. Your mileage may vary. Etc.