How are fees determined in the Lightning Network?
Fees are determined by the node operators. It's a free market, you can set whatever fee you want. Of course there's lots of competition, so if you want people to route payments through you, you should set a low or no fee.


  If there aren't fees in the Lightning Network, what is the economic incentive to operate a node?


Even without fees, there is still an incentive to operating a node and using LN. In order to use LN, you must be a node. You will want to use LN to avoid paying higher fees with on chain transactions, and thus you will be operating a LN node.