It would be highly atypical for a license to stipulate such power as demanding access to your site, I mean who would trust it?
That said there are plenty of malicious code out there, especially if you are getting themes/plugins from shady distribution channels and/or pirated copies.
Also if extension supports updates then it does give vendor access to your site of sorts — in serving those updates to be installed on your site.
In a nutshell you should stick with reputable sources for your purchases. If you have high security requirements/concerns you should consider getting whatever code you use to be professionally audited for security issues.