That sounds like good old-fashioned market segmentation to me. People who aren't price sensitive will pay to check their bags. People who would rather save their money and have more hassle will cart it through security and then gate check it when the bins fill up.

I am puzzled by the various choices that airlines make that seem to dramatically increase their aircraft's turnaround time at the gate. I can only assume that either it doesn't do as much harm as it seems to, or short turnaround isn't as valuable as I think it is.