What if I steal the hotel shampoo?

It is theft, and if the circumstances are serious, it will be sentenced.

What the hotel offers is disposable or reusable. If it is disposable shampoo and shower gel, you can usually take it away. If it's a big bottle, you can usually use it several times and you can't take it away. It's not provided for free, and the price will be marked next to it, so you should read it carefully before you consume it.