The cue angle is an important part of consistency and you are right, a level cue deflects with no "swerve". I've had this conversation on more than one occasion, the "swerve" is from elevating the cue and/or pivoting, but when the cue is level it deflects and doesn't come back. This is why I recommend a slight angle for enhanced accuracy with 'The Touch of Inside'....it keeps the cue ball from deflecting out of control.
The pure deflection is what you're seeing, the spin isn't really making it curve more, it's just not stopping it so it continues (in the same direction through momentum), especially if you hit with an above average speed. Just like the example set forth by the masse', the more the angle, the more the spin will effect the line of the cue ball path.
On high speed shots using a level cue you actually align/aim at the same spot on the object ball as where your tip is contacting the cue ball.....this means when cutting the ball 15 degrees to the right, you actually aim/align to the right side of the object ball. This is the only situation that this happens from my experience, and one of those "secrets" that few player (except champions) "real eyes". 'The Game is the Teacher'