There's no such thing as "an extension to some program". A derivative work is a work that contains the original. Using the privileges provided by copyright law, the creator may impose licensing restrictions on how the original work is used - but that's contract law, not copyright.

For example the GPL and the AGPL define different sets of use restrictions, none of that matters in this case because the original work is not being reproduced or used per se.

As I already said in my other, down-voted comment - copyright is only about verbatim, or near verbatim copies, in whole or in part - it's the spirit that both judgment and the letter of the law are supposed to follow. Copying of functionality is not subject to copyright.

For example, one can use the same topic for a work of poetry for a similar aesthetic effect and that doesn't infringe other poems.

The GPL used a hack to stretch copyright law into a near opposite but stretching it further goes into absurd territory, achieving the opposite of what the GPL claims to protect.