In the source code of the uvm_mem configure function you can find this code:
if (m_access != "RW" && m_access != "RO") begin
`uvm_error("RegModel", {"Memory '",get_full_name(),"' can only be RW or RO"})
m_access = "RW";
end
And the documentation of get_rights is:
// Function: get_rights
//
// Returns the access rights of this memory.
//
// Returns "RW", "RO" or "WO".
// The access rights of a memory is always "RW",
// unless it is a shared memory
// with access restriction in a particular address map.
//
// If no address map is specified and the memory is mapped in only one
// address map, that address map is used. If the memory is mapped
// in more than one address map, the default address map of the
// parent block is used.
//
// If an address map is specified and
// the memory is not mapped in the specified
// address map, an error message is issued
// and "RW" is returned.
//