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I haven't heard of the kerb issue, however in the UK we have regulations for speed bumps that are a maximum of 100mm high. Some in certain areas are just 45 degree ramps up to a flattened plane and then a 45 degree ramp down. With suspension compression there isn't much space for contoured tyre spats that don't bend or conform or lift in some way.
Vekke, I don't think that is the case entirely, that the reason they are vertical is so you can drive up to a kerb. Because they could make them any shape as long as they don't extend below 100mm.
I think the reason is, given the restrictions of needing to drive up to a kerb, the flat plate gives the lowest total drag.
Using a flat plate creates downwards and sideways air movement, giving the effect of a larger deflector but at the expense of higher deflector drag compared to a sculpted contoured deflector.
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