Ha, come on, is this oppression? And the problem we all have is that this guy is taking it out on the clerk, who has no responsibility in deciding this, has no control, and present a reasonable solution despite not having too.
The guy who went there for ice cream is more to blame than the clerk in this issue.
First, yes, it is. It's enforcing a social norm without any actual need (by society at large, I do understand the motivation of the icecream shop). Note that I am not saying who is responsible for it, oppression can be institutionalized and without any bad intentions by anyone in particular, that does not make it any less oppressive to those who are affected.
Second, how did you get the impression he was "taking it out on the clerk"? He simply was asking for an alternative method to access the WiFi and expressing his concerns about his privacy. Are you also to blame for taking it out on the clerk if you ask, for example, whether you might open some window because you think it's too hot or too cold or whatever?
The guy who went there for ice cream is more to blame than the clerk in this issue.