It's not but people don't seem to realize this or maybe just don't want to think about it. Spending $100B to produce 1GW of power (made up numbers) is not an economical source of power. So hydrogen being free (deuterium and tritium are essentially free; Helium isotopes are more complicated) is irrelevant until the capital cost of the plane is much, much lower.
And even then you still have to deal with these significant issues:
- Neutron embrittlement of the container;
- Energy loss from the chamber from neutrons; and
- Containment. The plasma is essentially an extremely high temperature turbulent fluid. Because of the turbulence and the super-high temperatures containment is likely to remain a significant issue.
I hope fusion becomes commercially viable and economical but I'm just not convinced (yet) that that will ever be the case. It certainly won't be ITER even with tens of billions spent on it.
People get caught up on the fact that stars do fusion without considering what's different. To summarize:
- Energy loss from neutrons is essentially a non-issue because of gravity and just the size of stars. To put this in context, it's estimated that photons created at the Sun's core take ~30,000 years to escape;
- Stars are relatively inefficient with their fuel. IIRC the Sun converts ~4.5M tons of matter into energy every second. It sounds like a lot but that's a tiny fraction of the Sun's mass (~10^30 kg). That's because hydrogen atoms are so unlikely to fuse and they go through several intermediate states before that happens. Fusion in the lab already produces many more fusion reactions per unit mass than stars do.
I firmly believe that space-based solar power collection is our most likely future.
EDIT: corrected "photons"