Learning to play and read music depends some amount on intuition. If you've ever heard someone say that a mathematician's downfall is reading music, it is because there is open room for interpretation when reading it, whereas mathematics usually has only one form of doing it.
That being said, the standard method you yourself refer to is agreed upon based on this simple inference: If you draw the bow to play one note, the next note must change direction, in order to maintain balance in the amount of bow available. This is something that either must be explained in a lesson, or is present as text earlier in the book. Otherwise, the explanation is not complete.
Starting from that, once a string player sees example 1, the implication (that should have been learned) is to do a down bow, then an up bow, then a down bow, successively.
As for your variable example 1s, let's tackle them in order:
1-1 requires all down bows. To continue bowing multiple notes on the same down bow requires a different notation. Here, as the player you must be instructed to retake the bow at the frog (or near it) for each note for all notes where the down bow sign is. Otherwise, you'll end up playing either staccato (separated notes on a single down bow), or a tie (the down bow makes the notes flow into one another).
1-2 assumes the "standard method". It clues you in with the first down bow. Lack of signs means that you follow the standard style of bowing, which means to change direction with each note. To verify this, another down bow sign is introduced at the start of measure 2.
1-3 is an interruption of the standard method. The first four notes (on the 1st measure) are bowed in the standard method of down-up-down-up. Measure 2 begins on an up bow, so that means you should retake your bow to the tip, and bow upwards towards the frog, effectively reversing your bowing to up-down-up-down.
1-4 is explicit about bowing in the standard style. You must bow down-up-down-up in this order.