Digital platforms are not only match-making intermediaries but also establish internal rules that govern all users in their ecosystems. To better understand the governing role of platforms, we study two Airbnb pro-guest rules that pertain to guest and host cancellations, using data on Airbnb and VRBO listings in 10 US cities. We demonstrate that such pro-guest rules can drive demand and supply to and from the platform, as a function of the local platform competition between Airbnb and VRBO. Our results suggest that platform competition sometimes dampens a platform wide pro-guest rule and sometimes reinforces it, often with heterogeneous effects on different hosts. This implies that platform competition does not necessarily mitigate a platform's incentive to treat the two sides asymmetrically, and any public policy in platform competition must consider its implication on all sides.