Should I include interviewees in my dissertation acknowledgement?
I would think that it is probably enough to recognize them as a group: Thanks to the fifteen people whose ... enables this research. You might want to name the companies, provided that there was some cost (monetary or otherwise) to the companies. 

Alternatively you might, at the end, without connecting any individual to their responses, thank them by name, maybe obfuscated: J. Doakes, M. Jones and thirteen others who prefer not to be named. 

Likewise, if the positions held by the respondents is essential you might list that with or without names, even if it is just summarized: Three executive vice-presidents, one chairman of the board, and ... (That isn't the same thing as individual acknowledgement, of course, but informs your readers.)

My reasoning is that you probably don't want readers of your thesis to contact your responders, more to avoid them being bothered than for any other reason. 

If the research requires that a name or other identifying information be associated with a response then you need explicit permission. You likely need to retain the written permission. 

I'll also note that there are laws in various places that speak to issues like this. In the US, most universities have an office of research that will give you very detailed information about research, especially research involving human subjects. It may be the same elsewhere. In the US, see an officer of the IRB (Institutional Review Board) at the university. Your research may be covered or not. I suspect that for simple interviews it won't be covered, but you should check.