This is the way Numbers works.
When you select any cell on a table, that table becomes active. An active table displays its row and column reference tabs.
A Numbers table may be locked (as may other objects).
A locked table cannot be moved, nor will it accept any input.
A cell is part of a table, not an object, and cannot be locked (except by locking the table).
Numbers recommended practice includes separating input cells and other (fixed data and result cells) onto different tables, and locking the tables containing no input cells.
One means of protecting cells on an unlocked table is to place one or more transparent objects (eg. a Shape) in front of the cells needing protection, and locking that object. There are shortcomings to that method, and I have not seen a recent recommendation regarding using it.