Tesla may have pushed too far, too soon
To be clear, that's not the exact verbiage Mobileye is using. The company did, however, point out that the current system isn't designed with anything remotely resembling fully autonomous driving in mind -- and that includes the semi-autonomous Autopilot that Tesla so staunchly stands behind. It's concerned about its reputation in light of all the negative attention Tesla has been garnering recently. As Mobileye itself told Ars Technica, "there is much at stake here, to Mobileye's reputation and to the industry at large."
The company is what's considered a tier-two supplier, which means it works directly with the biggest companies you've never heard of to supply sensors and systems to a veritable slew of cars you do know. In other words, Tesla is a small fish in the very large ocean in which Mobileye swims, and any potential damage to its reputation caused by a company like Tesla pushing the systems beyond their engineered intent is simply not worth the risk.
Mobileye wants to go fully autonomous anyway
There's a certain irony here, in that Mobileye is possibly ending its relationship with Tesla over the degree to which Tesla's Autopilot system takes over driving control, while at the same time preparing to develop fully autonomous systems in a recently announced three-way partnership with BMW and Intel. Furthermore, indications are strong that it's in the later stages of negotiating a similar deal with VW (which, obviously, includes Audi, Porsche, Bentley, etc...). The aim is to get cars on the road that truly don't need a driver in order to be safe. Such a project, done to a level of safety that can satisfy even the most conservative of lawyers, requires a level of patience that Tesla, or more specifically, Elon Musk, has never shown.