Splunk does not care what port you want to monitor - it can be anything. However, Splunk may be restricted because you are running Splunk from a non-privileged account (as you should).
On Linux for example, ports under 1024 are considered privileged; a non- root user cannot read these ports. This may be why you can't monitor port 80. There are ways to get around this.
I am also concerned when you say "monitor port 80 for traffic." Splunk's network inputs (TCP and UDP) read data from the port and index it; is this what you mean by "monitoring"? I feel like this is not what you want. You might also want to look at the free Splunk App for Stream