I must admit that this was the first thing that occurred to me- rival owners scaring away punters. I doubt also whether the true nutcases will need or want any special knowledge of how to inject something, they'll do it anyway, just because. That's assuming of course that this stuff is actually happening and there doesn't seem to be much reliable evidence of this.I don't think the intent necessarily has to be sexual - I suspect there are people out there who simply get off on the sense of power involved in removing function from another individual. There are also cases of spiking being used to undermine the reputation of bars and restaurants etc. (In fact, my old boss was spiked while out for a meal in central London some years ago. He has asthma - which may have exacerbated the issue - and actually ended up hospitalised, and although tests proved the presence of some nastiness or other, the police could not go much further than telling him that they believed the attack was part of a war between rival restaurants.)
Also, I believe that the spiking individuals for sexual purposes tends to take place in more intimate spaces - where an individual has been invited back to someone's home, house parties, hotels, etc. And you can see - from a purely practical standpoint - why that is likely to be the case.