You experience ‘relief’ from something that is uncomfortable or onerous; therefore the phrase ‘tax relief’ implies that tax is always bad — indeed, that it constitutes a ‘tax burden’. When their opponents reduce taxation, liberals prefer to speak of tax ‘cuts’ (usually ‘for the rich’). A ‘tax relief’ scheme can also be used as a form of tax ‘avoidance’, which is the legal form of tax ‘evasion’.