I do of course agree with you that you can't generalise - but many (most?) businesses do actually generalise by deciding they can't possibly share any changes they have made regardless.

As for SAP customisations - I am sure you will find these should in general be considered to be "content" rather than code - as I am quite sure you won't get you hands on SAP's source to make mods - customisations are generally rule-sets and are not really changing SAP per se.