The fact it is incorporated into the state codes doesn't suddenly INVALIDATE the copyright. However, it has been held that such incorporation is not valid unless it provides open access to what is effectively now enacted law.
You'll find now that nearly all the model codes have free non-commercial access even while maintaining copyright to protect their ability to being subscribed to by the local authorities.
I can tell you are not a lawyer. At least some lawyers (and law libraries and others) still do buy statutes and case law in printed books. There are some materials that are just easier to work with in that form. Moreover with statutes having a library of not just the current statutes (which are easy to find online) but past years too can be very valuable. I have nearly two decades of the Internal Revenue Code (IRC) in hardback form. Those statutes are extremely well laid out, include numerous notes and references not available online, as well as an incredible in depth index. Since I often have to work with prior years of the Code when representing clients in litigation and in tax controversies with the IRS having those prior years available is extremely valuable. I also have paperback versions of the current code and regulations so I can flag, highlight, and make notes directly in them which makes finding the information I want much easier. That's just part of my print library. Of course having electronic access to legal materials has cut down considerably on the print library I need to maintain today, but it has not completely eliminated the need for some materials in print.
I recall two decades ago predictions that by now computers would drastically reduce our need for paper as the need for paper documents would be totally eliminated. (Computers can't do much to eliminate our need for toilet paper. ) And yet today we use more paper than ever before. Computers have eliminated use of paper in a number of areas only to create new uses of paper in other areas.