Same in the US, they are often fronts for prostitution and other organized crime. Here in the US (I live in a deindustrialized former industrial zone) the derelict downtowns are also full of bail bondsmen, payday loan sharks, DUI lawyers, dollar stores, as well as delicious cheap ethnic food (like the kebab shops in the UK).

Not sure exactly what the previous content was referring to, but many salons (not just nail salons) also sell some of the products that they use (or claim to use) or knock-offs thereof, sometimes "under the counter". I can't see this making enough to cover high street rents though, however good the kick-back is, but it can't hurt too have an extra revenue stream.

they're saying they're drug fronts / money laundering