Off-label use isn't unusual.
Some of the drugs inhibit reverse transcriptase (an enzyme), so it makes sense for them to be active against multiple RNA viruses.
Typically, DNA is used and an enzyme follows it along to make RNA, and it's RNA (specifically mRNA) that is then used for an enzyme to produce proteins. Viruses don't do this on their own - they use cells to do this work for them.
With RNA viruses aka retroviruses, in our cells, the RNA is converted to DNA and this is done by reverse transcriptase which comes from the virus.
There are differences in enzymes from one species to another in general, so the reverse transcriptases might respond a bit differently to the different drugs, but the active sites would probably be very similar, so the drugs should have a certain level of effectiveness for most retroviruses.
Just noting this isn't true of all of the drugs, but some of them would be based on this.