Probably because if you are searching the table, you are searching for one
(or more than one related things - the AND is part of the WHEN syntax)
There is only the EQUAL condition allowed - either it is there or it isn't.
If you are trying to get an OR, you need multiple SEARCH ALL statements;
one for each of your different OR conditions.
but, enough guessing on my part why you asked the question.
If it was just a theoretical question, I don't care,
but if you have a specific requirement, state the requirement and someone maybe able to offer a solution.