I am no expert, but from what I understand this would be part of the operator inter roaming agreements with each other. Since this is normally restricted to temporary or casual traffic (90 contiguous days usually) the operators see this as low risk to their networks providing the devices have at least been certified for industry standards like PTCRB. In a lot of cases permanent roaming is prohibited.
Each roaming agreement is different, and obviously the biggest carriers have the best roaming agreements, generally. In some agreements, permanent roaming is allowed on operator Y as long as the number of devices for that customer is less than x% of the total devices deployed on operator X. So if you have 10k devices luanched on operator X, it might be ok to have less than 1k devices permanent roaming on operator Y. But if the number of permanent roamers goes over the agreed to percentage, then operator Y can refuse service for those devices (rare) or ask the company to change to operator Y SIMs if they are permanent roaming on their network.