They are trying to claim that you and only you can own that product. If you decide to move on and sell that art then you cannot. In other words you only have access to the art. It will have no value to a future owner.
However, there are court cases happening right now that could affect this position. A recent case was ruled that the owner has first-sale doctrine and can sell whatever he wants irregardless of a shrink wrap license.
That covered in our News section in case you want to read more about it.
Here's a link.
As to artwork and where to get it, you have a lot of choices but not what you might expect:
1. You can look for ready to use designs where you will do little more than size it, print it and install it. You might mess with the color but what we're talking about is a fixed design intended to save you as much work as possible. The problem with it is that anyone else can buy the same design and offer it to your customer. You have placed yourself in the position of selling a commodity instead of an original design.
2. You can look for ready to use design elements to incorporate into your original designs. Things like seamless tiles, textures, and various vector ornaments. The elements save you time and often serve as idea starters without taking away ending up with an original design.
3. You can look for the tools to create your designs from scratch and take the time to learn to use them. You won't be as efficient but your designs are going to be totally original.