Well, that is just my view of the relationship between some clients and myself (and I meant "assume" in the sense of "take onto myself" rather than "presume").
For example, I have a substantial client that I do consistent work for which includes marketing (note that the original poster does not mention anything other than design work).
I also have a domain name which I plan to develop in the client's industry. But I have not yet developed it because I feel there could be a possible conflict of interest. (As others have pointed out, the development could also be advantageous to the client, but in this case I have other projects to work on.)