Xavier,
Making a live copy is not the only way to infringe on copyrights.
Creating copies (what your software does) IS IN FACT copyright infringement...
even if these are not republished or distributed, if they only live in
somebody's PC... they're still copies, which if done without the author's
permission is illegal.

You guys need to research a little on copyright law, just to understand your
responsibilities having created this software.

I believe you should have a prominent notice on copyrights available to your
users. And I believe your software should not override robots.txt - no matter
why.

The more I read through your forum, the more I see from users questions that
this tool is unfortunately  used in most cases to infringe on people's
copyrights. I find users here asking why they can't download someone's PHP
script, or $50 web templates, and I find it sad that you won't take
responsibility for this and at least try to do all you can to make your users
aware of what they are doing.. and at least try to err on the side of the
law.

If one of those libraries or users want to copy my site for a legal use, I
prefer them contacting me directly, explaining me what they're doing and
asking. I explicitly allow some web preservation sites to grab copies of my
site. I just have a problem with any user, any time, making whatever copies he
wants without asking - and you making it so easy for them.

If you really want to offer a good tool to those users that would use it
legally, you would do all you can to prevent abuse. Users with good reasons
can always contact authors and request permission.