I hear rumors that Apple is improving its methods to limit the current you can draw from USB ports when you connect a non-Apple device. For future reference, how old is your MacBook?
That's probably a function of the USB spec and not a conspiracy theory.
USB spec says to only provide 100mA until the device asks for more. For USB 2.0 the max is 500mA. Apple's ports (like many PCs) allow for higher than that. It isn't called out in the 2.0 spec how to handle that. So the proper thing to do would be to limit current to 500mA.
I didn't weigh my words carefully. In their newest models Apple is less lenient than before in enforcing the USB specs for non-Apple devices. No conspiracy. I can understand their motives and have no qualms with them. I would like to add that this stricter enforcement doesn't affect the Arduino because it only needs 500mA, maybe just 250mA, which is granted by the USB specs. But I won't add that because I don't know whether it's true.
When I connect my UNO to my mid-2010 MacBook Pro I can go to the System Profiler and see that the USB device has 500mA available, 100mA needed. What if the Arduino, after successfully negotiating the 100mA, wants to draw more current? On my not-so-new MacBook I don't expect any problem. What about a strict specs-enforcer MacBook?
I may be wrong, but it seems to depend on whether the port is treated as a charging port or a downstream (data) port. Yes, it's a single physical port, but what it should do according to the specs depends on how we define it. As a charging port it has to provide up to 500mA, no fuss. As a downstream port, it is up to the Arduino to negotiate the new current before pulling it. Does the driver perform this negotiation? I don't know, but it seems it wasn't necessary in older models. Can the MacBook refuse? I believe it can. What are the externally observable results? I can only guess.