This is because in the example "sales_orders" data is being streamed, joined (using left join) to customers, and being appended to the silver layer table. When a sales_order comes in from a customer that was inserted some time ago (rather than in the current micro-batch being processed) the entire customer table has to be loaded to find that customer id and name. Therefore using LIVE.customers without "STREAMING" allows the join to be a stream-batch join (as described here).
Essentially because you only need the most recent records coming in from "sales_orders" you can use the "STREAM" keyword but the join requires the entire customer table to be loaded and hence the lack of the "STREAM" keyword there.
On the other side of the coin, you need to update the silver layer table only when a new sales_order comes in, not when a new customer is streamed into the bronze layer. That's another reason why you only need the STREAM on the sales_order table.
