Gaff has a very good point. Many venues that are "concert venues," or road houses that bring in touring shows are, more often than not, union venues. Also, Most venues that bring in touring rock shows or
theatre productions don't have a full time tech staff. The show brings it's crew with it, and then they hire what they need from the local union or non union pool depending on what they need. If you are looking for internships in theatre, you should look into local summer stock houses. If you are looking into R&R internships, well that is going to be tougher.