What are the legalities associated with mounting splicing wire connectors?
Mounting a splicing wire connector turns it into a pumpkinterminal block

We know that by definition, as per ZMVV.GuideInfo:


  Splicing wire connectors establish a connection between two or more conductors by means of mechanical pressure and are not intended to be permanently mounted. They are floating, such as a twist-on connector in an outlet box.


, a device listed to UL 486A-B-C as a splicing wire connector is intended to be free-floating in a box, not mounted to a surface.  When we mount such a thing to a box, though, it then provides a set of wiring terminals, with the mounting means  becoming a supporting block for said set of wiring terminals.  By definition, as per XCFR2.GuideInfo:


  This category covers assemblies of wiring terminals and supporting blocks intended to provide a means for the connection of wiring. These terminal blocks may employ terminal connectors, wire-binding screws, quick-connect terminals and similar friction fit terminals, which are intended for factory-wiring and/or field-wiring connections.


, we now have a terminal block, which as per UL 1059, is not rated for field installation, as the creepage and clearance distances (spacings) and short-circuit current ratings on terminal blocks must be further evaluated by UL in the end application.  Furthermore, it cannot be a power distribution block, listable to UL 1953, because as per QPQS.GuideInfo:


  A power distribution block consists of one or more terminal wire connectors mounted on an insulating base. Each connector has provisions for one or more incoming run conductors and multiple tap conductors. A tap conductor is of the same or smaller size as the incoming run conductor. The connectors may be of the lay-in construction, which do not require the incoming run conductor to be terminated.


, the individual connecting means in a power distribution block are considered terminal wire connectors, which are defined by ZMVV.GuideInfo:


  Terminal connectors establish a connection between one or more conductors to a terminal plate or stud, or to any similar device by means of mechanical pressure. They are fixed in position.


to be mutually exclusive from splicing wire connectors.

What can you use a terminal block for? Control circuits, that's what!

So, if you cannot field-install a terminal block into a mains circuit, what are they good for, then? The answer, as it turns out, is control circuits that fall under NEC Article 725.  First off, wire connecting means generally must merely be identified, not listed, as per NEC 110.14:


  110.14 Electrical Connections. Because of different characteristics of dissimilar metals, devices such as pressure terminal or
  pressure splicing connectors and soldering lugs shall be identified for the material of the conductor and shall be properly
  installed and used.


This, by itself, does not prohibit us from using terminal blocks for mains wiring.  What does prohibit that usage is the listing requirement for power distribution blocks, found in NEC 314.28(E)(1) (with similar wording in NEC 376.56(B)(1)):


  (1) Installation. Power distribution blocks installed in boxes
  shall be listed. Power distribution blocks installed on the line
  side of the service equipment shall be listed and marked “suitable for use on the line side of service equipment” or equivalent.


However, Article 725 does not state Article 314 is applicable (see 725.3 for details) to control circuits, so the 314.28(E) requirements for power distribution blocks do not apply to control circuits to begin with.

But, there is still hope!

If one wishes to join 14-10AWG mains circuit wires en masse in a tidy fashion, though, without resorting to the rather unnecessary bulk of power distribution blocks, there is still an option on the table though.  Some manufacturers make enclosed terminal block assemblies that consist of a UL 1059 recognized terminal block whose terminals are considered "suitable for field wiring" that has been factory mounted into a plastic or metal enclosure (junction box).  This assembly is then UL listed (as a trivial UL 508A control panel) as an assembly, which means that it can be used for field mains wiring without having to worry about the precise approvals of the terminal block itself.

Furthermore, while these boxes do not ship with grounding bars by default, one can add a UL 467 listed grounding bar (such as a Penn-Union NA-414) to such an enclosure to provide a central grounding point, thus saving the terminal block's terminals for current-carrying conductors (hots and neutrals), as per the Exception to NEC 314.28(E):


  Exception: Equipment grounding terminal bars shall be permitted in
  smaller enclosures.


Finally, if you cannot find a suitable assembly prefabricated/stocked, you can ask a local industrial control panel shop with the appropriate certifications to custom-fabricate such a thing for you and get it approved under their UL 508A listing, although at a not-inconsiderable cost compared to a prefabricated unit.