Weeds spread their seeds through the wind and attaching to clothing and the fur of animals. Once the weeds are carried off, they are eventually dropped and will normally find their way to the soil, take root and become established. If weed seeds are dropped on artificial turf, no matter how deeply they try and penetrate, or work their way down to the soil, there will always be another level of sub-turf to prevent them from reaching their goal. From the infill through the drainage network and leach bed, there are several inches of obstacles a seed must navigate through to be successful. In most cases, there is literally no chance of it reaching the soil below the artificial turf.
Weeds need water to thrive and grow. With artificial turf, water is easily funneled away along an intricate network of channels and grooves. This carries away the water, preventing it from pooling in low lying areas. Without a sufficient water source, weed seeds will not have what they need to germinate, nor will they be able to survive for any length of time without a form of irrigation. Even under the surface, the channels are designed to keep the water draining away from the lawn and into a drain or other type of water system.
In addition to water, weed seeds also need the nutrition they would receive from the soil. With no access to soil, they have no access to the nutrients. Even if a weed somehow managed to establish itself on the surface of the turf, its roots would not be able to penetrate the tightly woven backing that is designed to keep out the sun and slowly filter water through to the irrigation channels below. The only nutrients available would be what is on the surface of the artificial turf.
The tightly woven backing of artificial turf is designed to accomplish several tasks. It is woven tightly enough to keep the blades in an upright position. The backing is also designed to filter and drain water from the surface, deep into the lowest levels of the sub-turf. The tightness of the turf prevents weed seeds, sunlight and almost everything other than water from penetrating through it.
Weeds thrive easily under the right circumstances. Artificial turf removes the elements that are needed for weeds to survive, not to mention limiting the weed seed's access to the soil. Artificial turf manufacturers are so confident in their product's ability to prevent weed growth, that they include it in the product guarantee they provide to their customers.