I was denied ferry boat crossing from Shekou to Hong Kong airport...

I am a crew member with United Airlines. I arrived HK airport and took the ferry to Shekou without any problems. Upon my return to HK airport, after a couple of days in Shekou, I was denied ferry boat passage to HK airport because I did not have a boarding pass with a confirmed seat assignment on United Airlines back to New York. I informed the agent at the ferry port I was a crewmember with United Airlines and I fly standby and that my boarding pass is not issued with a seat assignment until I check-in at the UAL counter at HK airport. He stated I had to catch a taxi or a van to HK airport that I could not take the ferry across. Why is this and is there a way to get around it? Instead of spending 30usd for the 30 min ferry crossing I ended up having to pay close to 100usd for a van to take me to HK airport on a one and a half hour traffic ride with 3 other passengers. I am a USA citizen with a valid passport and a valid china visa. I don't understand why I was denied this passage. Please help me to understand and if it happens again is there a cheaper way to get across to HK airport from Shekou. Thank you in advance.