So they can sell you stand-alone data plans (for a laptop with an aircard) or tethering add-ons, at a higher price.

If you ask them why the price is different, they'd probably explain that laptops typically use more data per month than phone browsers, even though the cap is often 5GB for either plan. (Obviously, the internet tablets are a bit of a special case, as they're mobile devices with a mobile OS, and presumably low typical consumption, like on-device browsing, and you actually might not violate some TOSes if they specify tethering to a computer, and the IT is deemed not a computer...)

Of course, a real reason (which they won't mention) is to provide the same service at two prices, getting business customers (with deeper pockets) at a high price, and leaving the rest of us with a technically equivalent service tied to a less-desirable interface, but at a price that we'll pay.
Varies by carrier and phone, and as I use T-mobile, I don't know for any specific instances. In some cases, they just load a firmware that disables tethering, but especially with smartphones, that becomes impossible, and they use detection methods instead. I know some detections are made solely on the basis of data quantity.

Howardforums is a good place to get details, if you know what carrier you're interested in...

Yep. If you're in one of the metro areas where the T-mobile 3G is (or you have reason to suppose it soon will be), I'd definitely go that route. Otherwise, "You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya punk?"