/devilsadvocate on
On the other side of the coin, I think some mods should be gated. ComputerCraft, for example. On an open server that takes donations it is just asking for problems to give access to CC to unknown, untrusted, untried people. Asking for a donation for something as powerful and demanding to a server as CC is not too far fetched, imo. Basically, If the mod in the pack you are wanting to use would make your "footprint" on the server larger, what is wrong with asking for a donation? This is not to say that some SA's are unethical. I have seen some really shady donation tiers. I remember a server where you had to pay $25 in order to use a bucket of shit from MFR.
Modders put in a lot of work coding, no argument there, but the SA pays for hardware, bandwidth, and time to make his server runs properly, so I wouldn't necessarily say that ethical donation servers are "making money" off of modders. If server owners were not allowed to use popular mods/items as incentive to donate, do you think people (kids mostly) would donate enough out of the kindness of their heart to keep the server running? or would they just ride it til the wheels fall off? I understand the concept of setting your product apart to make people want to buy it, but I've played on very well run, nice looking servers with very lax donation restrictions, and they don't last very long.
but if people would learn to recognize a scam server when they see one, this wouldn't even be an issue.