Heh, the article itself is grammatically "interesting".
Also unusual about this word is that the past tense of the word. If you say a work is “copyrighted” that means is currently protected by copyright law, not that it is in the public domain.
If you say a work is "copyrighted", you're not using the past tense. You're using a past participle which can either function as an adjective "a copyrighted work", or as part of the passive voice "is copyrighted" (the latter being structurally ambiguous as it could also be an adjective).
However you categorise "is copyrighted", it's the word "is" that takes the tense (present tense) while "copyrighted" remains constant ("is copyrighted", "was copyrighted", "should have been being copyrighted"...)
If you use the past tense of the verb "to copyright", meaning - as the article itself says - "securing a copyright registration from the U.S. Copyright Office", then the verb behaves in exactly the same way as other verbs, such as "walked across the room".
What is true is the act of copyrighting a work results in the work's state of being copyrighted. That is that the particple of the verb can be used adjectivally. The author is correct to point out that it's like "protected", or "entrusted". It's also like "painted" or "broken".
Why would anyone think a copyrighted work is no longer under copyright protection? Do people regularly think that a broken vase is no longer in pieces, because the act of breaking it is in the past?
Basically, the difference between the verbs "to copyright" and "to walk" lies in the way they structure time. "to copyright" has an end-point, a result. Once copyright protection is active the act of copyrighting is finished, and the state of being under copyright protection is in place. "to walk" can - theoretically - go on forever. The end point is not implied in the verb itself. No obvious state that is given a name. Walking across a room certainly has consequences, such as - maybe - dirty footprints. These consequences might lead to a phrase like "the walked-across room," but you won't hear: "This room is walked across," if you're looking at the result; you hear "This room has been walked across" (present perfect, not past tense, by the way - oh, and passive voice, too).
Terms for this differ. One you might hear is that "to copyright" is a resultative verb, while "to walk" isn't. More abstractly, people talk about "telic verbs" (to copyright) vs. "atelic verbs" (to walk). It's a standard feature of lexical semantics and has connections to grammatical aspect (but not tense). It's not, as far as I know, unusual.
