A scam publisher offers me to publish, but it seems that I lose nothing if agree
(These comments are not intended to be specific to the named company, but to any that are generally considered to be suspect.)


By publishing with a company, you are partnering with them. Associating yourself with something you know to be dodgy is not a helpful contribution to your 'personal brand', in any respectable career.
It's unlikely they are being honest with you about the associated costs. They may not actually mean it when they say it's free for you, despite their advertising. Or, as suggested in comments, they may intend to use your work as bait to convince someone else to give them money.
Depending on the exact terms of the free licence you used, not giving them some copyright permission might mean they don't actually have the permission they need. If they don't need any permission from you, why did they bother to ask you rather than just going ahead? Alternatively, if you haven't used copyleft, perhaps they think they can try and limit what others can do with your work.