A university does not allow its employees to use its affiliation on papers done in spare time; how normal is that?
Generally the opposite is true in the United States -- the university wishes to have jurisdiction over all work done using university resources -- which includes the laptops and computers we use. Since faculty are exempt employees presumably we have no "free time," but instead work 24x7 for the university (except for the summer months for 9-month employees). So basically any idea that I come up with from August to May of the calendar year and that I use my school resources to work on, is the property of the university unless specifically disclaimed.

Just ask your patents and licensing office to sign a document saying that your university hereby releases you of all possible intellectual property and inventions that may come out of your paper (even if it's in History!) and see if they change their tune.



Note also that many American universities require their faculty to fill out a "Faculty Activity Report" every year that details all of their publications (as well as service/teaching work). The university uses it internally to determine promotions and retention; but it also forwards this data to organizations such as the National Research Council -- that determine university and department ranking using criteria such as number of publications.

Thus it is in the university's best interests (both for IP and for ranking purposes) to capture every single faculty publication that it can. So I would think your scenario would be implausible for most R1 universities in the USA, at least.