Collecting rainwater is illegal or restricted in some places primarily due to water rights laws and concerns about environmental impacts. In many areas, water is considered a public resource or shared property, and laws exist to ensure that rainwater replenishes groundwater and downstream water supplies rather than being diverted solely for individual use. Specifically:

• In states like Colorado, strict water rights laws historically prohibited collecting rainwater because the rainwater that falls on private property could flow downstream, where others have water rights. Although Colorado has relaxed laws recently, it still limits collection to certain amounts and purposes, such as outdoor use only.
• Restrictions are designed to protect natural water cycles, prevent harm to ecosystems, and ensure fair allocation of water resources.
• Some places regulate rainwater collection to prevent large-scale diversion that might reduce water availability for others or harm aquifers.
• Health and safety concerns also play a role, as untreated rainwater may contain contaminants.

Overall, collecting rainwater is usually legal or encouraged in many regions, but regulations vary widely by location and aim to balance sustainability, water rights, and environmental protection.