Ah, except that it wasn't. You can add any advantage to a power, and it's active when the power is. There's nothing tying it to the power. Furthermore, Ranged Attack is meant to apply to all ranged attacks. Accurate and Dangerous specifically work on the power they're applied to. And things like Improved Critical at least require you to specify what they're functioning on.

The bug here, in my opinion, was that it was flagging that it was the Slingshot power causing the error. I mean... it sort of is, in that you only get a warning when it's on, but given the program doesn't do any verification for powers that aren't turned on, it should have noted that your Thrown objects bit was what was outside of caps.