Red Hat has community and enterprise editions of it's application server. WildFly is licensed under LGPL. JBoss EAP is based on WildFly and costs a lot of money.
How is it possible that they don't provide JBoss EAP source code?
A bit more research shows that the previous version of my answer was based on an incorrect assumption.
JBoss EAP is actually also licensed under the LGPL license, sames as the WildFly product that it is based upon.
The LGPL license does not require that source code is provided to the general public, nor that the software is provided at no cost. The requirement is that the source code is provided, at no additional cost, to the customers that bought/obtained the software from you.
And the business model of Red Hat is slightly different from just selling software. They actually sell the software in combination with a support subscription. And without a valid support subscription, you can only turn to the (volunteer) community of WildFly to get your questions answered.
The support subscription is a regular contract between Red Hat and the customer and it is completely independent of the copyright licenses used for the software covered by the support subscription. This makes it uninteresting to try to obtain the JBoss EAP software from another source than Red Hat and there is no incentive to make an offering of the software.
TL/DR: The JBoss EAP source code is provided, but only the the customers of Red Hat that bought the software (along with a support package), and there is no incentive for the buyers of JBoss EAP to offer the software to others.