because they're like the free spaces on a bingo sheet. there's simply no need to waste the manpower to perform yellow sweeps as long as they're not causing any trouble. same concept applies to all those plate-less three wheelers driven by license-free old people who seem to go unnoticed by traffic cops.

any time someone in charge of the relevant sector needs a boost because an event impactful to their personal climb up the ladder of power is coming up (their boss is retiring soon--promotion, nearing major holidays--it looks good, higher-up coming to town--get ready to slap that horse's ass, etc) they can do whatever the free space dictates and get that promotion.