First, please do not use the 'global' keyword here, unless you are really building an extension package and want this class to be exportable.
In the current ruleset, the scanner will not flag this issue (but this will change in future queries). Nevertheless this will raise flags in the security review.
Batch classes are not exempt from sharing requirements at all. If you think about it, running asynchronously or synchronously doesn't have anything to do with whether an organization's security policies can be ignored or not. This is a business logic question not a performance/governor limit question.
This does not mean that all batch classes must have sharing, only that the sharing requirements are exactly the same for synchronous and asynchronous apex: all entry points to the app, especially global classes, must have the 'with sharing' keyword without exception. To perform a privileged operation in an entry point, put it in a helper class that is marked 'without sharing'. If there is a business reason to have the class run without sharing, then document the reason. In all cases, try to adhere to the principle of least surprise.