At the time Apple was tiny with little money. It had sold only 50 Apple I's to Byte shop. Regis McKenna was way bigger than Apple. It was common for PR firms, lawyers, even landlords, to do work for free or a little stock in the expectation that the new startup would grow up into a big client. Some even did. Pro-bono doesnt catch the spirit; it was an ad hoc investment.
And it worked! Steve Jobs had quite the relationship with Regis McKenna as described in Steve's biography. Regis McKenna even helped shape the "antenna-gate" response.
