It's a paid network. https://siasky.net/ is free for the moment, but will be switching to a freemium model in the near future. As a portal, siasky will have the ability to ratelimit and block abusers.
If you "abuse" the network using your own node, you will have to pay for everything you upload and download, which means the people storing it won't really think of it as abuse, you're just another paying customer.
If you "abuse" the network using your own node, you will have to pay for everything you upload and download, which means the people storing it won't really think of it as abuse, you're just another paying customer.