To recycle

(a) They throw the bottles into / in the recycle bin.

(b) They throw the bottles into / in the recycle bin to recycle them.

Thanks, Can I say,

They throw plastic bottles to recycle.

No. They must be thrown somewhere.

I still can't catch it. Why we must say, throw ....(sth) to somewhere? Why don't say it for the purpose? CAn give me some examples to make me clear? Thanks

It is simply that we throw most things somewhere; we don't just cast them into the air:

I threw the baseball to my brother.
I threw the banana peel into the garbage.
I threw the cat against the wall.

Mister MicawberI threw the cat against the wall.

Offensive example, Mr. M!

OK. I really threw the cat at my wife, but I didn't want that to get out.