In the Commentaries, Caesar writes:
Horum omnium fortissimi sunt Belgae, propterea quod a cultu atque humanitate provinciae longissime absunt, minimeque ad eos mercatores saepe commeant atque ea quae ad effeminandos animos pertinent important
So, I am guessing he is using the subjunctive with
commeant because it is an explanation or reason for the primary thought, however, in that case shouldn't he also be using the subjunctive with importo and write "importent"? Instead he just uses the normal active "important", which does not seem to be parallel with commeant which is subjunctive.