In Jainism, seaweed is generally avoided because it is considered to contain or support many subtle living beings, and consuming it can involve ahimsa violation.

Main reasons:

• Many microscopic lives may be present in seaweed and in the water it grows in.
• It is a water-based living growth, so harvesting it can disturb numerous jivas.
• In strict Jain practice, food is chosen to minimize injury to the smallest possible number of living beings.

So the prohibition is based on the Jain principle of non-violence and extreme care in protecting life. In some traditions, the exact level of restriction may vary, but many Jains avoid seaweed for this reason.