It can be. Providing the tool to do something illegal, with the sole purpose of the tool being to do something illegal, is illegal. Check out the YouTube DL project history/controversy.
Browsers provide the tool to save those pages (ctrl+s), in what way is it different? Honest question, as I'm building something that could be impacted.
Youtube-dl had a takedown because they specifically ran a circumvention in their tests. They removed the test and were restored.
Youtube-dl is still freely available.
Saving web pages is not illegal nor prohibited.