If you don't think this is reasonable, chances are you've never run a large website, or analyzed the logs of a large website. You'd be astonished how much robotic activity you'll receive. If left unchecked it can easily swamp legitimate traffic.

Unless you have a way for me to automatically identify "honourable" scrapers such as yourself as distinct from the thousands upon thousands of extremely dodgy scrapers from across the world, my policy shall remain.