The curious case of the disappearing cats

A few months ago, a cat disappeared from another second-hand English bookshop in Madrid (the store in question Petras, is only 5 minutes from OffSide).
This case was discussed on the MadridMan forum here, but no conclusion was ever made, and the cat was not re-found.
At the time I thought this may have been a kidnapping carried out by someone with a grudge. But now it transpires that another cat in the neighbourhood has also gone missing. The owners have put up a poster asking for help in retrieving the cat.
In both cases the cats were normal tabby cats.
Why would anyone go around stealing tabby cats?