I don't believe there is anything in law which prevents anyone providing anyone else with an interest free loan. But what the regulations did was to accept that sponsorship deals with related or "associated" parties were revenue which had to be proven to be at FMV whereas loans from shareholders were not APTs and no FMV tests needed to be carried out on the interest rate on these loans. This made the APT regulations breach competition law. Furthermore this special treatment of shareholder loans was at the request of a PL chairman/executive who had made a sizeable loan to his club, .and was thus a knowing and wilful breach of the rules and it made the APT regulations unlawful.
Martin Samuel believes that the PL seems to think there is an "air of criminality" around clubs which try to attract money and this seems to explain their attitude quite nicely. It is scared that some clubs might buy the title year after year and these are clubs with no experience of buying the title year after year, and so every penny these new clubs make is undeserved and must be carefully regulated. Some other clubs can be trusted to regulate themselves.
And now it's all come crashing down, leaving one hell of a shambles but of course we all trust the PL to clear it up. After all they know now what's lawful and what isn't. Or they should. Or ....