I haven't looked into the why so I can only guess a reason based on my experience.

My experience is this: I created an LNC connection string that only has permission to create invoices and read info from my node.

Then I published that LNC connection string on one of my test stores -- my "Alice" store.

Then I pretended to be a customer -- "Bob" -- and I used the LNC connection string to connect to Alice's node and load an invoice. It all worked as expected so far: I saw a nice, shiny invoice and I could display it as a qr code for Bob to pay.

Then, while Bob was still viewing that invoice, I pretended to be a second customer -- "Carol" -- and I tried to do the same thing Bob did: I tried to use the LNC connection string to connect to Alice's node and load an invoice. But it did not work. I got an error that said something like "the connection is already in use."

My guess is that the LNC devs were very motivated to ensure that each LNC connection string is only used for one service. I think they don't want two devices to both have access to the same node via the same connection string -- they want some sort of "separation of strings," so that you give a new string to every service.

My guess is this: to help ensure that users do not share a string between two devices, they handicapped the strings so that they cannot establish two simultaneous open connections. Consequently if Bob is using it, Carol cannot.

Therefore, if I used LNC, Bob could effectively bring down a store's lightning capability by using up its only available connection. He could just get an invoice and constantly keep a connection open to check its status, without ever intending to pay. I don't want that so I don't want to use LNC.