Those parking gates were installed in advance of the expanded Metro line that will eventually reach Dulles. There will be a few Metro stations near the Tyson Corner center, and there will be no allocated parking for Metro customers. The gates are to keep future Metro customers from leaving their cars inside the hotel parking. I was told originally that they would not start charging for parking last January for another 1-2 years when they put up the gates. In March, they decided to start charging for parking.
Recently, they have started to waive the parking charges for weekend stays. I was told of this about two weeks ago when I checked in Saturday night. I'm guessing the parking gate goes down Friday and does not go back up until Monday. I am not sure when it goes down since I usually check in on Saturdays.
As for parking in the mall overnight, you can forget about it because the mall has security cars that patrol the area. Any vehicle that is not gone by 2 am will get towed, and you will still owe the tow driver $25 even if you show up before he tows your car away. I learned about this charge the hard way when I parked at an adjacent lot to a strip mall where I was shopping (that is illegal in VA and your car can be towed). There is no street parking near this hotel.
There is one hotel nearby that charges for parking (Hilton Garden Inn Tysons Corner- $5/day), and they have been charging for at least the past 2-3 years based on my memory.
This property offers a free shuttle for a 2 mile radius which runs until 11 pm each day. It is a great benefit if you stay around Christmas time when the mall turns crazy, and it can take 30 minutes to get into the parking lots and find a space. In that case, you can spend $6 in gas getting in and out of the mall's parking garage during the Christmas shopping season.
If paying for parking bothers you, the Fairview Park Marriott is about 5 miles away. Parking is free, but they offer no free breakfast to platinum members on weekends. Also, the room rate is $10-30 more per night on weekends, and they seem to charge more during the week as well.
I like the Tysons Corner Marriott because they have an above average offering in the CL, and they try hard to take care of guests.