Differentials - Differences: Non causality in the system
Replacing the derivative by forward differences is just a way to define a transformation from the $s$-plane to the $z$-plane, and it has nothing to do with the causality of the resulting discrete-time filter. As an exercise you could try to do the same with backward differences.

Note that this method for transforming a continuous-time filter to a discrete-time filter is useless in practice because stability is generally not preserved, and the transformed frequency response may be drastically different from the original response.