It doesn’t force them to do anything, nor does it ask them to waive any of their rights (which is often illegal and/or unenforceable). Instead, it asks them to certify that they are not subject to laws more restrictive than those in the US. If they are, they are not allowed to register. As the site owner, you have a legal right to rely on your users not lying to you. Your slavery example is an entirely different scenario - you are asking people to waive rights they have (to not be a slave in this case). That’s not what this checkbox says.

The main point of the checkbox is to signify your intention to not offer services to people subject to the GDPR or other restrictive laws. We have been advised (by actual attorneys) that this should meet the standard built into the GDPR that we do not “envisage” the offering of goods or services to those subject to it.

I think that at the end, world will be better place due to GDPR, but there is surely some rough ride ahead - not due to respect of privacy but due to violating it so often that it became normal to us.

I think that much greater threat is comming from a direction of US companies you use than from EU courts this (again, might) become another "patent trolling"-like action from some US companies.