I thought the same until someone shared some additional insights with me.
So basically for device verification to work, you have to prove to someone that you're an adult, typically by linking your real ID. The problem comes from when you log in to a porn website and they try to determine you're an adult by reaching out to that trusted 3rd party. Now even though the porn site doesn't know who you are, only that you're an adult, the 'trusted verifier' does know that you've visited the porn website. This makes that organization a huge security risk as it directly links your identity to visiting controversial websites.
Who would you really trust with that info? Corporation or government, both have major risks to collecting that info. What happens when FL bans porn and starts targeting people they know have accessed it via this database? What happens when LGBT info is labeled 'adult only' and requires this tech to access, creating a database of potential 'undesirables'?
Once it's created it's absolutely positive that the data will be hacked and that the government will use this mechanism to target at risk groups.
The difference between this and in person ID checks is one of data persistence. Bars and such things just look at your ID, but don't typically log it in a database. Compiling a persistent database of every 'adults only' only action is just too risky.