Asking if the position is tenure-track.

In many cases, they might think it is clear from the job title. In the US, job titles with the words "Lecturer", "Instructor", "Adjunct", "Visiting", "Clinical" are almost always non-tenure-track.

The job title "Assistant Professor" is usually tenure-track, though with some exceptions: "named" assistant professorships like "John Doe Assistant Professor" may be effectively postdocs. But it does not hurt to ask for clarification.

As David says, talk to someone in the department instead of HR. The HR office is usually not closely involved in a faculty search and isn't likely to be the best source of information.

It is not impolite to ask. Usually the tenure-track and non-tenure-track descriptions are a major consideration of academic job postings. I find it a little odd that any job posting would neglect to mention this.

Getting in touch with the department chair or search committee (or other responsible faculty member listed in the solicitation) might be a better option than contacting the HR department.