In Jain Dharma, grains are not considered completely violence-free. Eating them does involve some jīva-himsa, because many small living beings are harmed during farming, harvesting, storage, and cooking.

The reason they are allowed is that Jainism recognizes a difference between:

• avoidable violence, and
• unavoidable violence in worldly life

For householders (śrāvaks/śrāvikās), complete freedom from all violence is not possible while living in the world. So Jain ethics teaches:

• minimize harm as much as possible,
• avoid food that involves more injury when possible,
• eat with awareness and restraint,
• reduce attachment and greed.

For monks and nuns, the discipline is much stricter, and they follow more severe limits on food and preparation to reduce violence.

So the point is not that grains are “non-violent.” The point is that Jain Dharma accepts only the minimum necessary violence for survival in worldly life, while continuously encouraging ahiṃsā, restraint, and detachment.

In short:

Grains are allowed because absolute non-violence is not practical for a householder, but Jainism still treats their use as a limited form of violence and encourages minimizing it.