There's fairly little spam, because the providers are happy to disconnect people for any sort of automation. So most companies are a bit skittish about taking on people that might be doing that, as they don't want to lose their connection.

In fact, it's a bit surprising Twilio's still able to offer such things. They routinely violate the rules that apply to the rest of us in the industry. Usually you've got to keep a 1:3 in:out ratio per number. Not sure what position Plivo's in. In light of the ratio requirements, maybe free inbound SMS is a way to help out on that.