Using someone else's wireless service is indeed stealing, as it involves using a resource that someone else has paid for without paying for it. It may not always be totally unethical, however, especially if you talk about it with the person, and in some communities, anonymous strangers may actively promote the use of their wireless by leaving the network unprotected and inviting people to use it.
When someone orders wireless service, he or she must invest in a router and be prepared to pay monthly fees for access. Using someone else's wireless service may be free for the user, but not for the person who owns the account. While it can be tempting to take advantage of an unsecured network to save money, there are some disadvantages to using someone else's wireless, both for the owner of the service and the user.
For owners, the clear disadvantage is that when people piggyback on their networks, it eats up bandwidth. This can make Internet service slow for the people who are actually paying for the network, which can be frustrating. In addition, if the service provider has a bandwidth cap or allowance, the service may be cut off if the allowance is exceeded, or an additional sum may be tacked onto the bill. Furthermore, people using the network could potentially access other computers on the network, if their users have set their computers to share data, and this could compromise the integrity of the machines themselves, as well as the data stored on them.
For someone who is using someone else's wireless service, the primary disadvantage is that the service could drop out of cut off suddenly, and the owner of the network may decide to boot piggybackers off the network through the administration software. It also leaves one's computer potentially vulnerable, as detailed above. Furthermore, in some regions, people can be prosecuted for theft of Internet service, and they may be required to pay fines, perform community service, or be penalized in some other way.
Using a wireless network that someone else has set up isn't always necessarily wrong, however. Sometimes, neighbors band together to share service, for example, in which case the bill may be split. When someone is having problems with his Internet service, he might ask a neighbor if he can use hers for a few days while the problem is resolved. In other cases, people who believe that Internet access should be available to all may leave their networks unsecured so that other people can access them.