Are "terminal block" style junction boxes allowed in the US?
I'm pretty darn sure the answer is "no"

Non-metallic boxes, as well as oddly shaped/sized metal boxes, in the US are required to be permanently marked (labeled) with their "official" volume in cubic inches so that the correct box fill calculations can be run for them.  This is Codified in NEC 314.16(A)(2):


  (2) Other Boxes. Boxes 1650 cm&sup3; (100 in.&sup3;) or less, other
  than those described in Table 314.16(A), and nonmetallic
  boxes shall be durably and legibly marked by the manufacturer
  with their volume(s). Boxes described in Table 314.16(A) that
  have a volume larger than is designated in the table shall be
  permitted to have their volume marked as required by this
  section.


I seriously doubt that IEC-style splicing boxes would carry such a volume marking, rendering them unusable in the US then and there. Furthermore, the size of that box is similar to a North American pancake box, which lacks the volume needed for tapping even a 14AWG NM cable, by Code. (At most, you can splice two 14/2 cables together in a pancake box, or use that style of box in its normal habitat, namely hosting a dead-end light fixture.)

As a result of this, I seriously doubt these are at all usable in the US, and that's before you get to the fact that the one you've linked is likely cheap Cheese-pipeline garbage that's not even suitable for use in a UK installation.