Let's think about the case that a master issues a exclusive write transaction to a slave.
On the AXI Specifiaction document, it says that if the slave doesn't support "Exclusive Accesss", then it will always update the target.
But what about the response?
If the slave respond with "OKAY", the master may think that target is not updated. ("OKAY" is error response for the exclusive write transaction)
But, as slave doesn't support "Exclusive Access", it can not respond with "EXOKAY".
How can slave handle this case?
The AHB5 protocol allows an exclusive write without an exclusive read, but it immediately says that this exclusive write must fail and HEXOKAY must be deasserted, so it would be a pointless thing to do.
The AXI spec in this area then seems more sensible in making this an illegal thing to do if it must always fail.
So in AHB5 it is legal (but pointless), and in AXI it is illegal.