The answer is - possibly - yes, unless you have explicit permission to access the network (such as a free WiFi hotspot at the airport or town square).
Many states have criminal laws that prohibit accessing any computer or network without the owner’s permission. For example, in Texas, the statute is Penal Code section 33.02, Breach of Computer Security. It says, “A person commits an offense if the person knowingly accesses a computer, computer network or computer system without the effective consent of the owner.” The penalty grade ranges from misdemeanor to first degree felony (which is the same grade as murder), depending on whether the person obtains benefit, harms or defrauds someone, or alters, damages, or deletes files.
The wording of most such laws encompass connecting to a wireless network without explicit permission, even if the Wi-Fi network is unsecured. The inclusion of the culpable mental state of “knowing” as an element of the offense means that if your computer automatically connects to your neighbor’s wireless network instead of your own and you aren’t aware of it, you haven’t committed a crime. But if you decide to hop onto the nearest unencrypted Wi-Fi network to surf the Internet, knowing full well that it doesn’t belong to you and no one has given you permission, you could be prosecuted under these laws.
The federal law that covers unauthorized access is Title 18 U.S.C. Section 1030, which prohibits intentionally accessing a computer without authorization or exceeding authorized access. But it applies to “protected computers,” which are defined as those used by the U.S. government, by a financial institution, or used in or affecting interstate or foreign commerce. In addition to fines and imprisonment, penalties include forfeiture of any personal property used to commit the crime or derived from proceeds traceable to any violation. You can read the text of that section
Clearly, a line is being drawn by statute about this issue. Hopefully this information helps you to decipher when you should and when you should not connect to an open WiFi network.