Calling B2D a subset of SaaS doesn't make a huge amount of sense.
Sure, there are some great SaaS products where developers are the main choosers/users, like Twilio, GitHub, HipChat and Parse. But the amount of time developers spend working with SaaS products is still small compared to the amount of time we spend working with local apps (e.g. vim, Visual Studio, git), programming languages/frameworks, databases, software systems (e.g. SugarCRM, Unreal Engine, Hybris) and of course PaaS/servers (e.g. Linode, AWS, Heroku). These are just as valid sectors of the B2D market as SaaS offerings.
Sure, there are some great SaaS products where developers are the main choosers/users, like Twilio, GitHub, HipChat and Parse. But the amount of time developers spend working with SaaS products is still small compared to the amount of time we spend working with local apps (e.g. vim, Visual Studio, git), programming languages/frameworks, databases, software systems (e.g. SugarCRM, Unreal Engine, Hybris) and of course PaaS/servers (e.g. Linode, AWS, Heroku). These are just as valid sectors of the B2D market as SaaS offerings.
Case in point: at SnowPlow (https://github.com/snowplow/snowplow) we are squarely in the B2D market, but are not a SaaS business.