I think you are misunderstanding.
First off so there is no ambiguity let me say clearly that GlobalSign's policies do not allow the use of certificates that chain to our roots to be used for MiTM purposes (or other malicious use cases for that matter).
That said there are lots of reasons why a company might want or need to operate their own in house PKI and have that PKI trusted; one easy to understand case is google; though they are not our customer they operate their own in house PKI that they use to issue the many many SSL certificates their infrastructure utilizes:
https://sslcheck.x509labs.com/en_US/sslcheck?host=www.google...
But SSL is not the only reason one might want to operate their own internal PKI that is trusted, other cases include business to consumer and business to business communication (for example S/MIME).
As for how we (and other CAs) go about ensuring trusted root deployments are not used for MiTM cases, there is the obvious contractual obligations that are accompanied by audits but we also were the first publicly trusted CA to use Name Constraints to deploy qualified subordination to these deployments (which has become a requirement now – it was not when we started).
This approach technically limits what domains these enterprise trusted root customers are trusted by mainstream clients for - see: http://unmitigatedrisk.com/?p=24
There are also a set of “Trusted Root” customers who are in the process of becoming trusted by browsers (which can take 3-5 years) where we will cross-certify their root enabling them to be trusted while their own key material becomes ubiquitously trusted.
I should also add that Mozila’s root inclusion policy (as well as Microsoft’s and the other root programs) allow for these scenarios.
I hope this helps clarify, please let me now if you have any questions.
Ryan Hurst
CTO, GlobalSign
ryan.hurst@globalsign.com