Communist parties are meant to be organizations of action rather than simply representing an ideology. They are supposed to do stuff. When there is a natural disaster in China for example, they often send out the CPC members to help with disaster relief. You are actually expected to participate in things so naturally you have to be a bit picky.
A person does not have to be a member of the party to view it positively. People generally view their fire department positively despite not being firefighters themselves. As long as the party is doing good work people will view it positively even if they are not themselves a member of it. In fact, letting in people willy-nilly can hurt the party's reputation if it negatively impacts their work.