Substantial anti-trust concerns are avoided, while building exactly the moat you want: no competitor can afford to compete with the level of integration investment.
Thats if they keep ARM alive and mostly like it is for now at least.
Having some advantage may be worth the purchase.
Example: Nvidia would prefer certain specifications on an internal bus they'd use, but nobody else uses as a standard.
There doesn't appear to be sufficient profit margin in the industry for too many greenfield, non-reference custom implementations.
So if Nvidia-ARM sticks out a reference implementation or architecture choice that's beneficial to Nvidia, there's limited capability for others (Apple, Cavium/Marvell, Qualcomm, & Samsung?) to strike their own path, and they'll certainly be paying steeply to do so.