Unfortunately (if you are the user), fortunately (if you are the entrepreneur), even works that are in the Public Domain can be protected against duplication, display, derivation, distribution and public performance.
Physical inaccessibility is one type of protection given to public domain materials. For example on one of Todd Bolen's archeological digs he uncovers a scroll of the Book of Genesis written by Moses himself. This scroll is public domain because it was written before 1883, but Todd is practicing the time honored tradition of "finders' keepers". He's got it locked in a vault. If you can't get to it, you can't use it!
Licenses and contracts are another means of protecting public domain intellectual property. Licenses and contracts always supersede copyright. Online Indexes and databases have very explicit licensing agreements because much of the data is public domain and it's the only way they have of protecting their market. On the bright side though, as an innocent third party, unless you have clicked or signed one, you are not responsible for adhering to the license agreement signed by another party.
Encryption is also used to protect material that is available to the public domain. At the moment anything protected by encryption or by some type of technological protection the work cannot be copied because it would require the breaking of the code. It is illegal to circumvent any technological controls protecting access to a work even if the work protected by these measures is in the public domain. The same is also true of fair use -- it is infringement to circumvent any access controls even if the work can be defended as a fair use.