It’s not the charge money part. It’s that it charges money and then still grabs and sells toms of your data. That’s something the Lemmy crowd is super opposed to even in free products, but a subscription to have your data sold when the service itself (Lemmy) is hosted on a donation basis and does not cost the devs a cent to use is too much.

Most of that opposition is directed at the Sync devs of course, not against some user who has made the decision that the proposal is good and decided to use Sync.