Tesla very obviously negotiates with each of its SpC landlords the rights to emplace the charging site. That may be on a full-site basis, it may be on a per-stall basis, and so on. The one site in which I played a role - and this is very early on (2013) - the rate was $50 per month per stall. I have no knowledge of any current transactions, although I can find out what the Soldotna, AK one is, were it to be important.
At any rate, if we make the assumption that Buc-ees management and by extension their franchisees are competent in business, which their success suggests is the case, it is likely they are extracting from Tesla some rent, rather than, for example, paying Tesla for the ephemeral possibility that SpC users will be filling their tills with enough purchases to make that sensible. BUT - were my logic to be the case, then one would think (this is: were I Tesla this would be one of my negotiating demands) that patrons of the site be treated equally, whether they were buying gas or buying electrons...i.e., be able to use the squeegees and cleaning fluid.