As to what you can legally do, the law looks at the terms of the license or the contract to which you have signed. The signing of publishing rights belong to the copyright holder(s). In the case of Unity 3D the run-time license remains the property of Unity 3D and does not transfer to the publisher or end-user.
Instead the publisher is using a piece of code that is still expressly licensed by Unity 3D for the purpose of running your published works. However, there is a problem with this model. If you bought a car and it had a term of condition that you could not modify the way it looks, or some advertising, then you would expect that you do not actually own the right of the transfer of that property aesthetically.
However, this is kind of the same thing when you buy a motor vehicle. The problem is that although the physicality of the situation stipulated you own the vehicle, you in fact do not have the right to claim you own the intellectual property of the design of the motor vehicle. You do however have the right to modify how your vehicle looks. We all be up in arms if there was a law that said, part of your motor vehicle's pricing model is based around the badges the manufacturer puts on the car. You cannot deface the badges on your car? Anyway, I for one fully acknowledge that there should be a credit in there, but not in the first scene of the application. Here is my argument why.
The market gets pretty saturated with crap apps. Or, crApps for short. The problem is with so many crApps that display the Unity Logo in the first couple of seconds of opening, give your application an automatic disadvantage due to prenotions of absolute low budget fruitfulness. A shitfest of waded bloated unoptimized crappy bogware that all bare the same opening logo.
Here is a better idea, choose a point where the credit / badge is shown. Maybe select on Application exit, or a point within the application life-cycle using a smaller placement or simply elect to remove the logo in return for real metrics. If the application calls to the mothership to let it know that the it is has been run, then they can start just focusing on collecting revenues based on the term of the license, rather than any potential advertising returns.... I don't know, maybe they do that already?