But it's arbitrage. The market rate is $4000 but the current tenant is paying $1000 and subletting for $3500. The inconvenience is worth $500/month, and you can't just go directly to the landlord to cut a better deal because the apartment is already "occupied" by the tenant paying $1000 who the landlord can't remove.
I'm talking about a subtenant retaliating against a prime tenant who kicked them out. Getting the prime tenant evicted is easy. On average, they're easier to find and collect from. Suing to take over the lease is hard, but collecting from the landlord is easy.
Having subleased to a legitimate person before, it's already annoying/stressful enough to be a sublandlord since you are taking on a lot of credit risk as is. If I'm in a position where I'm going to shadily rent out my below market apartment I'm not going to rent it out to someone that a normal landlord otherwise wouldn't pick.