The short answer is that we don't know why this isn't allowed. Xero have not currently enabled this functionality for UK users. This means no apps can upload payslip adjustments to Xero.
Australian and New Zealand (which has an almost identical API) users have this functoinality enabled, so we would very much like Xero to turn it on for users in the UK.
The fastest way to make this happen is to let Xero know directly or by voting at the link below (Xero use this to determine development priorities).
Please click here to vote