If you purchased licenses to a product, the manufacturer makes the rules, or you negotiated the rules together via contract. If you bought a "product" they can build it on whatever platform they want and determine license compliance by whatever means they want. It would be unfair to not tell you how they audit that, but it sounds like they have done so. There are a lot of "what if"s with their approach to licensing (if I login during a backup, do they count RMAN backup processes as using a seat?) so you may need to have a conversation with them about "here is the implication of your licensing implementation and how it is/isn't fair." Put in an enhancement request with the vendor. Does your license with the vendor include the right to access the underlying database directly via reporting tools? Were the reporting tools part of the ERP suite?
I used to be a DBA in the aviation industry. I'm surprised they can fit a usable maintenance product into the size and processor limits of 11g XE, unless this is for a very small fleet.