Maybe "IN" is a reserved word in Lucene.Net. Anyway, although for the end user the "search" interface might be the same, you should distinguish between searching and filtering data. There is a difference how things work in the background.
I assume that the job postings is somehow definite set of data. So, you can simply filter the data and not search the data. In cases like this, or with the product catalog, it might be easier to filter data rather than searching (with certain level of random results). E.g. when the user is searching for "green shoes" you can do a simple data filtering and add appropriate SQL WHERE condition.
Filtering takes an existing full list, and removes items based on criteria that match/don't match. While Search takes a blank slate and adds to it based on criteria that match/don't match. And in some cases, this could be tricky and some unwanted items may fit the criteria.
If you are using portal engine, I would recommend considering re-designing this and use the data source + filter + listing web part approach.
If you are using MVC - I would use e.g. the DocumentQuery and/or MultiDocumentQuery methods, compose appropriate WHERE condtions and return just the pages/items which wit your conditions.