Not all 1-Wire devices have to to be powered from the data line; so-called "Parasite Power"
All iButtons do - because they inherently have only two contacts, but other devices are available in packages that permit a "conventional" separate power supply line.

The Data Sheet for each specific 1-Wire device will tell you about its specific requirements in this regard, and how to meet them.
eg, if you parasite-power the DS1820 temperature sensor, the Data Sheet tells you that you must provide a Strong Pullup for the duration of the conversion process.

The Application Note that I cited originally also convers these issues - like I said, you really must read that one first!