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Some GPL software distributors do require the user to accept the GPL license during install.
How do you USE software without obtaining it from DISTRIBUTION ?
I can only assume those distributors themselves have just copied that practice from proprietary solutions without thinking. GPL is a copyright license, not an EULA per se, and is invalid when only using the software or code. GPL (and copyright law itself) applies only when distributing such yourself.
I give you that that it's a bad practice to ask users to agree or disagree with GPL before they can install the software in question; you are not required to agree with GPL to use GPLed software and thus the dialog is unneeded.