Large solar collectors allow the ability to harness truly massive amounts of energy, much more so than could be available from fusion reactors, on a watt-per-kilo of apparatus basis. It is not inconceivable that orbital solar collectors could be devised from monomolecular films that would be hundreds of thousands, perhaps millions or even billions of square kilometers in size. The materials for their construction would be easily available in their sun's asteroid belts, and the manipulation and construction energy would be negligible in zero-g.

Fusion reactions require specific isotopes which may not be available there in large quantities, or may not be scalable up to the amounts they may require. For that matter, we don't really know if electromagnetically-contained fusion reactions capable of really large amounts of energy output are even technically possible. We can only do it with fission triggers and even nature requires stellar-mass gravitational fields to do it.

Of course, its also possible that there are easy ways of producing large amount of energy that we are too primitive to have thought of yet. Its hard to speculate on what may either be impossible, or about which we are totally ignorant. Then again, energy production may have nothing to do with it. They may have engineering projects which we can't even conceive of yet.

And always, we run the risk of anthropormorphizing our ETs. We always ask the question "Could they do this?", Instead of "Why would they want to even if they could?"

Or maybe they are building a wormhole, and what we detect is just the scaffolding.