Why bed bug treatment usually falls on the landlord
In most cases, yes. A California landlord is generally responsible for treating a bed bug infestation, especially in a multi-unit building where the bugs spread between apartments. California law also places specific duties on landlords regarding bed bugs.
Keeping a rental sanitary and fit to live in is part of the landlord's duty, and bed bugs are very hard to eliminate without professional treatment. Because they spread through the structure, an infestation is usually the landlord's to handle building-wide, not something a tenant can solve alone. California's bed bug laws add requirements to inform tenants and act on infestations.
Responsibility can be argued differently if a tenant clearly introduced and then hid the problem, but landlords generally cannot simply push the cost onto tenants. If your landlord is refusing to properly treat bed bugs, it may be worth getting guidance about what the law requires.
This is general information about California tenant rights, not legal advice. Every situation is different.