You need Entry Credits to enter data into the Factom Protocol (some will provide these for free in Discord if you ask to get you started).  Also, you mine external data providers, so you do need to signup for one.

Mining always has external costs such as electricity, hardware, etc.  For PegNet, an external data provider will be one of those costs until pools become a thing but then their fees will of course cover those costs.

In the end, it all comes down to profitability.  Will it still be profitable to mine PEG despite the external data provider?  That's all that really matters...