Various hits for this. It seems to mean small/new companies can enter the market and take advantage of what has been previously built up/invested in by more established companies.

Here's an example:
The pharmaceutical industry has been angered by the growth in parallel importation, as this reduces its profits. It views paralleltraders as ‘parasitic competition’ in that they make a profit fromtheir trade but have not invested in any of the research for themedicines themselves
www.urchpublishing.com/publications/parallel_trade_exec_sum...

Though in Italian we only say "concorrenza parassitaria" an English lawyer would exhaustively call it "unfair and parasitical competition".

Hope it helps,
Carla