It's all about the lawsuits... and The Celtics are a juicy target - they have lots of money and can be easily embarrassed (i.e. extorted) with bad press since they are so well-known.
As Rabbuk says, the authority chain is a big deal, you can't have a relationship with someone who reports to you - appearance of favoritism at minimum. She gets passed over for something, she could sue. She gets something that someone else does not, they could sue.
The relationship ends badly, she can sue and claim it wasn't really consensual, or it became a power thing.
To avoid all this, there was probably a pretty clear company policy saying either "don't" or "inform the company immediately" to get it into the light. And he probably didn't follow that.