There are plenty of examples of commercial software that is distributed by the copyright owner free-of-charge, but requests agreement to a conditional license during the installation process.
I believe that in the US, this relies on a court ruling (sorry, no cite) that as the use of software inherently requires making a copy of it, such use is a copy right. This is unlike the case of a book or a painting.
There are also examples of commercial software distributed free of charge with no license agreement whatsoever. I sincerely hope that the decision that using software is a "copy" right not implicitly granted by its distribution is overturned, as the notion is truly ludicrous.
That doesn't necessarily mean that EULAs would be unenforcable; one could still argue that the license page of the installer is an effective access control mechanism between the user and the software, possibly invoking some twisted interpretation of the DMCA.
...and I hate that I feel like I have to put this, but as I mentioned elsewhere, IANAL.
I believe that in the US, this relies on a court ruling (sorry, no cite) that as the use of software inherently requires making a copy of it, such use is a copy right. This is unlike the case of a book or a painting.