Because, depending on how it's coded, the patreon API can be used to see who logs in, and the dev can block access at any time by simply revoking the rights. Some people aren't okay with that, since there doesn't need to be a reason provided. Post something on the patreon the dev didn't like? Revoke. Let a friend play the game without paying for it? Revoke.
Some people are just not okay with that level of access to their machine, such as myself. Even if it's only to 'slam the door in your face' and not remote access. Not to mention the number of vulnerabilities this opens up, which again, depends on how the dev implements it. Not accusing the dev of any malicious intent, but DRM on this game is just... A weird decision