The “minimum” set of features for an SMB VMS depends on the end user customer segment into which it’s being sold. For example, a retail convenience store will require a spot monitor and maybe POS integration. Whereas a small office building will not require POS but may desire integration with an access control system and LDAP.
A problem with playing the 'minimum features' game is that most DVR/NVR systems are not targeted directly to the end user customers but rather are targeted at the integrator/dealer channels that sell them. Those channels, as an aggregate whole, satisfy all end-user customer segments. As such, the dealer/integrator channels demand a recorder that supports all features for all end user segments. They want to be able to sell one DVR to both a convenience store as well as an office building. (I’m generalizing a lot here, but "beer" with me). So they want a DVR with all the features: a spot monitor, LDAP, POS integration, and access control integration. That’s why the recording systems you listed have lots of features.
None of that really matters though, because as John says, the real answer is cost. This is because the SMB customer does not use video surveillance as a mission critical system within their infrastructure. They do not use it daily, and do not want to use it daily. So they're not willing to pay any more than necessary just because something has exactly the right feature for them. Rather, it’s like insurance—they get the cheapest they can and just hope they never have to use it. And, like insurance, they're really all very much the same offerings, so they need no other differentiators than cost.