Actually, the crystal controlled requirement only applied to Novices. Newcomers who bypassed the Novice license could use VFOs from Day 1. The requirement helped keep some crystal manufacturers in business for a few decades. But it didn't work perfectly. Some Novices found out the hard way that the marked frequency of a crystal wasn't necessarily inside the Novice subband. And then there was the harmonic trap..... In the days when Novices had to use crystal control, none of the HF Novice allocations were harmonically related. It was easy to tune up using the wrong crystal or the wrong harmonic and cause havoc. Worse, the harmonics of the 80 meter Novice band were outside the 40 meter amateur band, and on an HF aviation band. Ancient history now. The crystal-control-for-Novices requirement went away almost 50 years ago.