To cut a long story short, that ACK was sent when the socket didn't belong to anybody. Instead of allowing packets that pertain to a socket that belongs to user x, allow packets that pertain to a connection that was initiated by a socket from user x.
The longer story.
To understand the issue, it helps to understand how wget and HTTP requests work in general.
In
wget http://cachefly.cachefly.net/10mb.test
wget establishes a TCP connection to cachefly.cachefly.net, and once established sends a request in the HTTP protocol that says: "Please send me the content of /10mb.test (GET /10mb.test HTTP/1.1) and by the way, could you please not close the connection after you're done (Connection: Keep-alive). The reason it does that is because in case the server replies with a redirection for a URL on the same IP address, it can reuse the connection.
Now the server can reply with either, "here comes the data you requested, beware it's 10MB large (Content-Length: 10485760), and yes OK, I'll leave the connection open". Or if it doesn't know the size of the data, "Here's the data, sorry I can't leave the connection open but I'll tell when you can stop downloading the data by closing my end of the connection".
In the URL above, we're in the first case.
So, as soon as wget has obtained the headers for the response, it knows its job is done once it has downloaded 10MB of data.
Basically, what wget does is read the data until 10MB have been received and exit. But at that point, there's more to be done. What about the server? It's been told to leave the connection open.
Before exiting, wget closes (close system call) the file descriptor for the socket. Upon, the close, the system finishes acknowledging the data sent by the server and sends a FIN to say: "I won't be sending any more data". At that point close returns and wget exits. There is no socket associated to the TCP connection anymore (at least not one owned by any user). However it's not finished yet. Upon receiving that FIN, the HTTP server sees end-of-file when reading the next request from the client. In HTTP, that means "no more request, I'll close my end". So it sends its FIN as well, to say, "I won't be sending anything either, that connection is going away".
Upon receiving that FIN, the client sends a "ACK". But, at that point, wget is long gone, so that ACK is not from any user. Which is why it is blocked by your firewall. Because the server doesn't receive the ACK, it's going to send the FIN over and over until it gives up and you'll see more dropped ACKs. That also means that by dropping those ACKs, you're needlessly using resources of the server (which needs to maintain a socket in the LAST-ACK state) for quite some time.
The behavior would have been different if the client had not requested "Keep-alive" or the server had not replied with "Keep-alive".
As already mentioned, if you're using the connection tracker, what you want to do is let every packet in the ESTABLISHED and RELATED states through and only worry about NEW packets.
If you allow NEW packets from user x but not packets from user y, then other packets for established connections by user x will go through, and because there can't be established connections by user y (since we're blocking the NEW packets that would establish the connection), there will not be any packet for user y connections going through.
Best Answer
Considered a bug in kernel 3.10 (maybe only in my architecture, arm64). Works fine in 4.x kernels, tested in few of them. According to kernel diagrams and docs routing is the same between kernels 3.x & 4.x and should work in both, but it doesn't.
br_netfilter is a separate module in kernel 4.x, you have to
modprobe br_netfilterto enable functionality.