These are just my opinions:
1.) The hotel provides soap and shampoo for its customers to use while they stay in the hotel. The cost to provide these amenities is NOT trivial.
2.) For sanitary reasons, once a bottle of soap or container of shampoo is opened, it cannot be reused and will be thrown out.
Therefore, one should only use the amount of shampoo and soap needed to shower. I think it is petty theft to hide or pack unused shampoo or soap each day (whether opened or not) so that the staff provides new bottles that they otherwise would not have provided. On the last day of the stay, however, any remaining unused shampoo or soap is ok to take, since it is effectively garbage to the hotel. If you are unsure about this, ask the front desk. I think the same principle applies for condiment packages in restaurants (e.g., ketchup packets). The condiments cost a lot of money, and are provided with the intention of use with the food you are buying right then from the restaurant – not to stock your kitchen.
As for the magnetic key cards, the hotel can reuse these and they should be returned unless you are told otherwise by the staff. I once accidentally took a magnetic key with me. I called the hotel from the road (wife was driving, don’t worry) to ask for their mailing address, and they said not to worry and to keep it.