Other than on something like standing room only public transport e.g. train, tube, bus, tram at peak times, how would some dodgy perv get his foot in position for shots without it being really obvious he was up to no good?
.. Though given the amount of time I unfortunately spend on sardine special trains (where actually having a seat is just a fantasy) I suppose perv has lots of chances for his dubious activities.
I assume the whole risk of getting caught / it involving "real people" (as opposed to pr0n actors) must be a major motivator - as others have said, plenty of imagery / vids on the web so most people could find whatever their particular fave peccadillo was. The Daily Fail sidebar of shame would (by all accounts) have kept him happy*
* I have no desire to give Mail any ad revenue so do not visit links to it, but its sidebar of shame and hypocrisy (upskirt pics whilst articles are full of censorious guff) is legendary (in my case I read about the sidebar in Private Eye as the Mail double standards are frequently highlighted there)