Seeing it again, it's not really the movie one as it is more softer material without all the leather straps etc.

But still clearly Wonder Woman

Matt, DC let Hyperion go back in 1969 when the Squadron appeared to be a one-shot and the precedents regarding copyright were not so onerous. When Marvel became more aggressively plagiaristic doubling the Squadron into two teams and using both on a routine basis, DC editorial offered a limp pastiche of the Avengers to counter the offense, limiting whatever legal response they might have been able to mount at that time. 

When Gruenwald decidedly to happily wade shoulder-deep into the waters of intellectual property theft with his Squadron Supreme mini-series, too much time had gone by since the initial offense for DC to respond. Marvel's legal department, however, insisted that Gruenwald change the nature of the villains he had oh-so-cleverly concocted for his story line (Wildcard to Remnant, Puffin to Pinball, and so on) because those would have been new transgressions and therefore subject to legal action. 

Sentry and Supreme do not contain enough visual or story similarity to Superman to warrant a Cease and Desist, or at least did not at the time of their inception. 

Devil's Due is clearly trading on DC's copyrighted and trademarked property in the same marketplace and format that DC markets Wonder Woman in order to sell their product. DC can not allow that to stand lest they lose the ability to protect themselves and their licensing of the WW visual in the future. Believe me, I wish they had shut down the Squadron Sinister back when Roy Thomas was slapping his knee and endangering his employers with the idea back in '69. 

Had Devil's Due gone with a Carol Danvers pastiche for their cover and Marvel had issued the letter, would we still be having this discussion, or is this just more DC-bashing here at the JBF?

Who is bashing DC here?? Are we not allowed to criticize a company for going after a parody cover? And for the record,  I, and others here were led to believe that DC was objecting to the line art cover. That cover is clearly parody. The painted cover is more questionable,  which I acknowledged above.

And, yes, I would have the same questions and issues if it were Marvel, or any other publisher.

Curious, Brian, who here do you think was posting simply to bash DC?

Also, the f*cking Fox News article linked to in JB's initial post is what gave me the idea that it was the line art version that was in question. Which is what I deserve for getting "news" from Fox.

It is clearly covered by parody. Remember, DC published MAD Magazine which does far worse. However, the costs of defending even such a simple clear cut case against Warners may be more than a New York comic store can rustle up.