They get round this by what's called "intercompany eliminations".

UHC and chums simply buy the medical providers, and can then use them to launder people's premiums. UHC et al can only keep a maximum of 20% (? - I forget the exact figure) of the premiums; but if UHC simply own the hospitals, then money laundered through those hospitals via very high profit procedures etc become permitted profits for UHC.