Why should they be voluntary donations? Why are you entitled to the work of another person? What a bizarre argument to make... You're saying that the mods aren't worth the money but are upset because they do have SOME value to you. No one cares that you're gaming "on a budget", because we all are. It's likely that the guy who is making the mod that you're currently getting for free is also gaming on a budget. This makes you sound like a spoiled brat.
I have a feeling you have no idea what Valve needs for hosting costs.
You have crafted up this boogeyman of the modding community who is going to come in, write a mod that is just tantalizing enough for you to buy, and then run off with the money. What if this change lets some of the popular modders go full time with their development?
I look at this as the natural growth of game development. Look at games with modding capabilities as "software platforms." The mods are plug-ins which extend the functionality. These plug-ins requirement significant development effort (whether alone or in a team.) Development is becoming simpler as tooling becomes more readily available and easy to interface with. These platforms offer assets and other items baked in that let individuals express their creativity. The last step is to get even more individuals interested as it could become lucrative*
This is a genuine question... Because I don't know the answer to it but it sounds like you do because it's the basis of part of your argument. When you buy a game, what are you entitled to? Do you get support for some period of time? Are you guaranteed any updates or patches?
It's a little bizarre to me to see all of this happening. It's almost like this "PC Master Race" bullshit has gotten into people's heads. Calling mods the "soul" of PC gaming. So PC gamers have been trumpeting their ability to "play free mods!" to their peers... and now suddenly, they feel it's being taken from them. It's almost like it's an attack on their values.