The jurisdictions of the companies are worldwide, and generally domicilied offshore (for the poker industry as a whole) where Monopolies considerations aren't a factor
If they were all domiciled in one country, then that market share would undoubtedly be seen as anti-competitive
Betfair is hugely dominant and is clearly abusing its dominance (commission much higher than main rival) and nothing was done when it was headquartered and regulated in the uk, so I doubt anything would happen if stars was in the same position.