The GM was interviewed by Seravalli on his podcast and directly addressed the reasons. The Caps had their own cap management tools but found that their staff were referring to CapFriendly quite frequently. They started asking themselves what would happen if that access suddenly went away, as there have been teams sniffing around CF for some time. They started to explore ways to bring in-house those same capabilities, primarily in terms of a user-friendly interface that their existing in-house tools lacked. The data was there but the ability to rapidly reference this stuff like you could on CF was lacking with their in-house tools. The interface is what made CF so useful, plus the additional value-added features.
Those conversations about building a site for internal use ended up involving talks with the folks at CF about how they do things and their processes, and eventually one of the ideas that arose was perhaps to buy CF entirely rather than pay someone to build a new site from scratch. So those talks matured and they ended up deciding to buy the site and hire the three main staff at CF to become Caps employees. Those three will not only maintain the existing site for employees' use on their Intranet, but expand the granularity of the site, integrating the data the Caps can access that the league maintains (but aren't publicly available).
It wasn't so much wanting to put other teams in a bind, as wanting to avoid being that party that ends up in a bind if CF disappeared overnight. It didn't end up looking any cheaper to build a fresh site and interface from scratch than to buy CF outright, so they did that and brought the CF team into the fold to run it.