My son happens to be learning Makos in school and we recently discussed the sugya (daf 15) of “lav she’kadamo aseh”. The gemara suggests that the category of lav hanitak l’aseh for which there is no malkos applies only in cases where the aseh can be fulfilled only after the lav has been violated. Lav hanitak l’aseh means the aseh serves to correct the lav; if the aseh applies even before the lav has been committed, it is not directly associated with correcting the violation. For example: the lav of entering the mikdash tamei is not considered lav hanitak l’aseh to the mitzvah of t’mei’im not entering the mikdash (vi’shalchu min hamachane…). The mitzvas aseh of keeping out of the mikdash while tamei can be fulfilled irrespective of whether one has violated a lav and entered the mikdash while tamei or whether one stands outside the mikdash and refrains from entering without violating any lav.
My son connected this discussion to our discussion of the mitzvah of tashbisu. The mitzvah of tashbisu applies both to cases where one has violated bal yera’ah and now has chameitz to dispose of on Pesach as well as to cases where one has not violated any lav and simply fulfills tashbisu by disposing of all chameitz before Pesach starts. If the Rambam holds that lav she’kadamo aseh is not classified as a lav hanitak l’aseh and is still subject to malkos, the Rambam's psak that there is a punishment of malkos for buying chameitz on Pesach makes perfect sense (I have also seen this sevara quoted in the name of the Sha’agas Arye but have not had time yet to read the piece inside).