Roaming with prepaid wireless phones is an interesting topic. Prepaid providers either allow or don't allow roaming via a setting on the handset itself. It's typically locked in, and can't be user selected on prepaid phones.
With postpaid providers that have additional charges for roaming, users can often set the phone to allow or disallow roaming, which allows control over whether or not they'll encounter roaming fees.
You can sometimes find prepaid carriers that also have roaming fees, and the good ones will allow you full control over whether or not roaming is allowed on your phone. This is either done by prompting you on your phone before you roam, or a global setting that allows you to disable roaming. That said, for the most part it's a prompt instead of a setting, as most people won't know/learn about the setting; i.e. it gets too complicated to explain/educate. when such a setting does exist, it will usually be defaulted to disallow roaming.
Either way, prepaid wireless carriers know that if they charge for roaming, and you don't have control over when you roam (i.e. knowledge as to when you're roaming), that you'll be pretty upset when your balance disappears!
Many prepaid carriers simply don't allow roaming at all because they need to pay their roaming partners, and it can get expensive, and may not be cost effective depending on what plan the customer is on. Rather than have customers upset about incurring extra fees (even when they know they're about to be charged), they simply disable it.
One good example of roaming is Sprint. Postpaid Sprint customers can roam for free on Verizon's network. This allows Sprint to get some extra coverage is rural areas where Verizon has better coverage. They don't charge customers for this, but do pay Verizon. Having better overall coverage is still worthwhile. However, Sprint does not allow its prepaid brands (ex. Boost Mobile, Virgin Mobile, and Assurance Wireless) to roam on the Verizon network. This not only saves on costs, but also provides some differentiation between coverage on its postpaid service versus its prepaid programs.
MetroPCS and Cricket is another good example. In 2009 they signed a mutual roaming agreement allowing their respective customers to get better coverage with no additional fees. In turn, both prepaid carriers save boatloads of money not having to build out their own networks to get better coverage. Note that in April 2010, Cricket announced that it is offering free nationwide roaming via roaming agreements with Sprint and Verizon (primarily Sprint I believe). This kinds of begs the question as to whether or not roaming on MetroPCS' network is needed, but that's a different discussion altogether!