Aren't there a dozen of bugtrackers, intranet collaboration software, CI tools and git hostings that they could download and install? What's so special with the Atlassian products?
those sucks a little less than, say, bugtraq, offer enterprise support on premises which you'd never use but you need to do the purchasing when you go for a big company and kind of have a big recognized name for their customization support even if their whole stack sucks.
I believe this is the reason why they use Atlassian[1] products, where rest of us would use trello, e.t.c.
[1] company that created jira