Can you just charge your phone/ipad/laptop at the airport at any outlet?

It's not theft. Unless otherwise noted, intentionally blocked, or obviously intended for some other purpose (e.g. to plug in an ATM), the outlets in the waiting area are specifically there for passenger convenience.

In fact, airliners and airports are specifically expanding this functionality for more people to take advantage of;

Omaha's Eppley Airfield has also been adding electrical outlets in various areas of the terminal and has more installations planned in the near future. "We regard them as a necessary amenity to users of the terminal," said Stan Kathol, director, finance and administration for the Omaha Airport Authority, "The traditional building-wall outlets only are no longer sufficient in today's society."

Furthermore

When the $907 million expanded Terminal 2 opened at San Diego International Airport this past August, the USB-enhanced power ports (more than 1,600 in total) at every seat were among the new amenities touted. "It's been tremendously popular," said airport spokeswoman Katie Jones... Just last week, Huntsville International Airport installed a half dozen new charging stations

Several airport spokespeople and directors are qouted in this article, touting their airports increase in the availability of such power outlets (and creating power stations to add to the already available power supplies). Clearly, if airport spokespeople and directors are touting the use of such power outlets, the airports intend for passengers to use them.

Even if wall outlets in airport seating areas were not originally placed there for passenger convenience, if an airport has taken no measures to block or otherwise discourage the use of such outlets, that should be a pretty good indication to you, that they are ok with it (or at the very least, that they don't care).