The location field is mandatory in bill form even if settings are set to not make the field mandatory.
Once Multi-Location Inventory Feature is enabled in a NetSuite account under Setup > Company > Enable Features > Items & Inventory then the Location field is set to mandatory by default in transaction forms like standalone Vendor Bills, Customer Invoices, and Cash Sales.
This is to facilitate the computation of Inventory related data like Cost of Goods Sold account and Inventory account balances and Inventory Quantities per Location.