After being mostly grounded for two years, Americans rushed to book summer flights this year — and are paying very heavy prices for their tickets. Airlines plead an imbalance between supply and demand. A shortage of pilots. Soaring jet fuel prices. Those factors are real. Yet airline analysts now see something more deliberate at play. As people have begun to balk at the sky-high prices, and some switch plans away from flying, demand has softened. The airline response is not to lower the fares, but to keep them high by further cutting the supply of seats. “They’re starting to see some resistanc…