Actually for a second rounder it has absolutely nothing to do with the salary cap. Second rounders generally get the minimum, and if a team leaves the roster spot blank you still need to place a minimum cap hold for the purposes of the cap.
Teams can only have 15 players on the roster.
Generally, a college player has to make the 15 man roster in the next year or the team loses his rights. Not the case for Euro second rounders, who do not need to be stored on 15 man rosters. So you can let them grow -- they could get better, and become a trade chip or a player... or they just disappear.
In the current format it just makes more sense to draft a Euro instead of a college player. You try to bring a couple decent college players to fight for the last roster spots as undrafted free agents.
All this being said the stashed second round euro's are not taking jobs away from Americans or college players as they are not on the 15 man rosters anyway.