I pay $200 a year ($17 a month) for access to a large, productive portion of VMWare's datacenter stack through VMUG. 6x that for access to a social media API is a terrible deal. At $100/mo, Twitter is basically saying "We don't want your business".
That is more likely the case here. They don't see profit from their side to make lower level access between the free and $100/mo tier as worth it... and would rather see corporate backed developers who are likely to have 5+ figure annual contracts than deal with tens of thousands of $10-20/mo subscribers disproportionally using the system.
