[pdq]

Their claim is only that Inspeqtor needs to keep the previous license.

    We will withdraw this takedown notice if and only if [private] and
    Inspeqtor adhere to the GNU Affero General Public License 
    which Monit is licensed under. This means;

    Our copyright notice is retained in the derivate work, and ALL 
    the derivate work is licensed and provided to users as “open
    source” under the AGPL. This include the so called pro features 
    of Inspeqtor which are closed source and licensed under a
    proprietary license.

Translating languages of a GPL codebase and then removing the license is not within the spirit of the license. Otherwise I can translate your code from C to Go, remove the license, then translate from Go to C, and voila, license-free code.

[icebraining]

The spirit of the license is irrelevant, since the question is whether the license applies at all. Assuming it is in fact a clean-room implementation - and I have no position on that regard -, it doesn't.

[olefoo]

"Translating" a program from one language to another is not a straightforward mechanical process.

In US copyright law and most of the countries that adhere to the Berne Convention there is a concept of a derived work. What the legal standard is varies, usually a translation of a work of literature would qualify, but whether a "translation" of a work of software would be considered a derived work is an open question that will not be resolved without litigation.

But it certainly seems that this takedown is bogus in terms of the DMCA since it is not a copyrighted work itself, but an ( allegedly ) derived work.

It certainly looks like an attempt to misuse intellectual property law to protect a product from functional equivalents that would not otherwise be excludable from the market.

[Rizz]

A derived work is still protected by the original copyright. Because there is a substantial creative contribution the derived work is also copyrighted in its own right, but that doesn't diminish the original copyright.

Translating a book into a foreign language is also a creative process, but the author of the original still has the right to say whether such a translation can be distributed and would expect a royalty.