No law, no obligation.
There is still no obligation for them to share a slot. Like I said, it is up to the motorist if he/she wants to share a slot.
How hard can it be to understand this "One vehicle, one parking slot". It doesn't matter if its a MC or a car.
Unless specified otherwise. So the MC's pictured earlier were definitely not parked incorrectly. If they don't want to share, then there is nothing you can do about it. Just find another empty space. If there is none, then the car park is considered full so park elsewhere.
Since you mentioned being "Considerate" as a point, I have a question for you. If you own a picanto, cherry qq, or a mini and the car park is full, will you be considerate and allow MC's that can fit in the empty space between your car to share the slot with you? Probably not. Will you allow another small car to park behind you and share the slot? Probably not. I wonder why?
I wonder what people will say if the SMART car comes to the philippines. You can fit 2 of them in 1 parking slot(one behind the other) with a little space to spare. Will people say that they are inconsiderate for wasting space for using just 1 slot each when 2 cars can fit in a single parking slot?
Here's an extreme example: (4 cars in one slot)
In the states there are parking meters for each parking slot. The occupant of the slot is required to pay or else his vehicle will be towed. Do you think motorcyclists will be sharing one slot? No they won't, so its not uncommon to see MC's occupying 1 slot each. Why should other people share my slot especially if I payed for it. Life isn't a charity case.