i have worked in a laboratory that "milks" spiders and the answer is no, it doesnt hurt the spider because they are aneathetised, with CO2 so they are totally unaware - i mean, lets face it, you'd be slightly mad to try and milk an angry spider whilst it is awake.
Venom is an amazing resource and it has a wide range of potential uses and is an up until very recently untapped resource for molecules to use in the fight against cancer, pain and many neurological conditions. So venom is useful for much much more than just anti-venom (which is pretty important if you live ina country where a spider bite may well kill you).
At present there are no restrictions on what you can do to a spider as it is an invertebrate and they are not covered by welfare legislation but i can tell you that the place where i worked (as a volunteer) put the welfare of the spiders absolutely first and has won awards for this, it is also campaigning to get stricter legislation in place.
The spiders here live a far better life than many pet spiders, sold in any pet shops. They have the correct environmental needs in place as much as is practical and they go about their day to day life, being spiders, and sometimes they are "milked". It is not in the company interest to over milk these creatures so they don't. Their venom is used for research purposes rather than anti-venom and they often work with species for which anti-venom hasn't been developed because they are mentalists so the work isn't without risk. It does however have amazing potential benefits.
A spider's needs are pretty easy to provide for, somewhere to hide, food and water - much less so than mammals and their sentience is at a lower level (in my unqualified opinion), however I feel that legislation to ensure the welfare of these wonderful creatures is hugely important.
I speak as someone who up until two years ago would have shreiked and jumped on a chair if a spider entered the room.