Parsing/typechecking inserts NOP

When parsing a sequence of length 1 consisting of an instruction i, Script_ir_translator.parse_instr produces a sequence of length 2 starting with i and followed by a NOP. Here is the incriminated code fragment

These NOP instructions slow down the interpretation of Michelson scripts. Removing them seems to work just fine, as done in !348 (closed).

Am I missing an invariant or a security aspect that would justify these NOPs?