Why wouldn't they work?
Each neuron's output is equal to a function over the sum of all its weights multiplied by their corresponding neurons. If that function is the Sigmoid function, then the output is squashed from $[0,1]$. If the entire layer uses a SoftMax function, then the output of all neurons is squashed from $[0,1]$ and their sum equals 1. In other others, they represent a set of probabilities, where you can then use cross-entropy to optimize their values (cross-entropy measures the difference between two probability distributions).
ReLU and ELU are simply other types of functions, whose output is not limited to the range $[0, 1]$. They are differentiable, like other activation functions, and so they can be used in any neural network.