It would be worth inquiring a bit more deeply into this matter, since the mere absence of a written rule does not necessarily mean that it is not an actual rule of the canteen. (There are many genuine rules of conduct in the world that are unwritten, or are written somewhere you don't easily find them.) The simplest thing to do here would be to contact the canteen manager (rather than the department staff) and inquire as to whether there is any policy or practice of serving faculty before students. You should also find out if the canteen is run by the university, or if it is a separate business entity contracted by the university.
Once you have established whether this alleged faculty preference is a real policy of the canteen, you will then know where to direct your criticism. If it is a genuine policy of the canteen, and you object to this policy, you have the ability to raise the matter with the canteen and the university, and ask them to change this practice. (Welcome to the wonderful world of political lobbying.) If, on the other hand, this is not a real rule, then it appears that the professor in question is either labouring under a mistaken belief in some special privilege, or he is just making it up. Either way, you then have the ability to raise the matter directly with this professor, or with his department, and request that he not cut-in-line at the canteen. You would also have the ability to make a complaint, and you could then back this up with communications with the canteen showing that there is no genuine rule or practice of this kind. Given that many universities/faculty members hold themselves out as holding very egalitarian political beliefs, it is also potentially somewhat embarrassing to conduct such a practice.
It is not completely unheard of for businesses to have preferential rules for service for certain groups of customers (e.g., movie theatres might have a separate fast-line for pre-bookings or special members). In the case of a university it is likely to genuinely be the case that the time of faculty members is much more valuable than the time of students, and hence, it is not necessarily irrational to impose a preferential-service rule in favour of faculty. Nevertheless, given that students are customers of the university, it is somewhat unusual to create an environment where the customer is given lower service preference than the staff. It is even more unusual if there is no signage to alert students to this practice, to mitigate their objections.
(If you decide to take this further, please post an update to your question to let us know what happened. It is an interesting matter, and I think many users of this site would be interested in hearing about your inquiries.)
Update: Based on the updated information added to this question, you now have a statement from the canteen manager to the effect that there is no policy of service preference to faculty. If you're interested in kicking up a stink then this sounds like a good lead-in to contact this professor or his department to me.