Would there be a cow to take if grass wasn't free.
Grass isn't free. On a purely technical level they do not directly pay someone to grow the grass, but from an economic perspective, there is a cost to maintaining the land.
If you simply set your cattle loose, they wander off and you lose them. So you have to fence in the land. Then you have to provide watering troughs. Additionally, you'll need to start partitioning the land.
If you simply let your cattle into the land, they'll tend to over graze and destroy the grass in one area, and leave the rest alone.
You must build fences to force the cattle to graze the land more evenly, so the grass is not destroyed.
Then you'll also need to do some land management, such as eliminating weeds that over run the grass. Sometimes you'll need to fertilize the grass depending on the condition of the soil, and the climate situation.
Lastly, you'll have to do some water management. If the cattle get into the water supply of the land too much, they can ruin the water supply and harm grass growth. So water inlets need fenced off, and water outlets need protected. You can't completely fence off the water, as it has to reach the soil to water the grass. But you can't allow the cattle into it, to ruin the water.
You may also need to determine what types of grass are in your grazing area, and may need to plant better grass than what is there.
Point being, there is a ton of work and effort, and even money, that goes into providing that "free" grass for your cattle.
That's one of the reasons that when you lease land for cattle grazing, you'll generally see two different prices for the same general area.
One owner is providing maintained land, which demands a higher price, because the land owner is spending the money to maintain the land.
Another owner is providing grazing land as-is, which fetches a lower price, because the cattle owner has to spend the money to maintain the grazing land himself.
The Bottom line again is... nothing in this world is free. Nothing.