Actually, it is allowed but not for end users (or only the biggest ones). Selected Google partners (parking companies, "content recommendation" platforms, zeroclick providers etc. can do arbitrage, i.e buying cheap traffic hoping that they earn better when redirected to parking pages.
This arbitrage is spreading like a virus and very bad thing for end user parkers. This is most probably the root cause behind RPC drops everywhere, because huge low quality referral traffic is sent to parking pages that increase the traffic supply for the same advertising budget.