You're not allowed to watch "something else" either - it's a national tax on televisions, not a BBC subscription charge.
Historically the detector vans tuned in to the local oscillator frequency generated by the television's "Heterodyne" tuning circuits. In order to tune to different channels, the TV generated different frequencies and used them to shift the broadcast an intermediate frequency to where they would decode it. If you know the intermediate frequency used by the TV (I believe they all use the same one), and can measure the local oscillator frequency, you know the channel being tuned to. You may well still know the frequency being tuned to with a digital TV, but each frequency now carries a multiplex of several TV or radio channels.
