In the case of an MTM tenant, you CAN evict someone for having a roommate. The law does not protect the right of an MTM tenant to have a roommate for more than 30 days.
There is no such law and a MTM tenant can't be evicted simply for having a roommate. A MTM tenant has all of the rights of a tenant with a lease. The only difference between a tenant with a lease and a MTM tenant is either the landlord or the MTM can terminate the monthly tenancy. If the landlord does not like that the MTM has a roommate he or she can elect to terminate the MTM tenancy, but no reason has to be given.
OP wrote:
Logically the solution to the problem would be to tell the tenant they are acutally saving money themselves by helping me out - a win-win situation right?!
There is absolutely no logic to that stupid illogical statement. How the hell is it possible that the tenant, who has to move and pay $200 - $300 more per month in rent, could possibly be saving money? It, of course, is not possible, despite the OP's twisted illogical attempt to justify her goal of evicting the MTM tenants.
Bottom line, as the OP's above stupid statement makes perfectly clear, the OP is a landlord trying to justify her goal of evicting her tenants. Regardless of the reason, this forum should not be helping the OP at all, IMHO.