Hey Juan, where in the spec did you find these numbers? When I browsed the thing all I found was the little 6.4.4 Prohibited Cable Assemblies section but no detailed explanation of the capacitive loading effect.
Another thing to note in that section of the spec is that they say "long" high speed/full-speed cables exceed the capacitive loading of low speed. They don't make an attempt to define "long" so we're kinda hung out to dry. That is unless they go into greater depth somewhere else in the spec or a peripheral document.
I guess this is a bit of a late reply, but I would say "long" is anything that is longer than the max length of the low-speed cable. I belive the full-speed cables can be up to 5 meters or about 16 feet. What is a short cable, 6 feet?
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I'm a little late chiming in on this, but thought I might point out that yes, according to the USB spec, a low-speed cable must be "captive," but captive is defined as either permanently attached to the device or using a connector other than a standard USB connector type to attach to the device. So a "captive" cable can be detachable as long as it doesn't use the same connector as a full/high-speed cable..
I believe the main reason why low-speed devices aren't permitted to use standard USB connectors is to prevent users from attaching cables longer than 3 meters. Low-speed cables have a limit of 3 meters, while full- and high-speed cables have a limit of 5 meters. (Since USB 1.0, I don't think these numbers are in the spec, but this is what they work out to according to the electrical specs.) If a low-speed device has a standard USB connector, a user might attach a 5-meter cable, which might not work reliably. So compliant low-speed devices must have either permanently attached cables or vendor-specific connectors.
As for cable sources, a search here will bring up many USB cable manufacturers:
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