I'm an undergrad math major and I'm taking a fairly advanced math class this semester which has pretty long and difficult problem sets. I've noticed that several students (probably most) use online resources (such as Math SE) for get help with the problems. A lot of the problems are difficult, but standard, results in the area, and so it's not hard finding proofs online. I will say that people don't blindly "copy" these proofs: they just look at them, understand them, set their computer aside, and reproduce the argument in their own words and understanding. I do this often too.
But I always feel a bit weird doing this. Isn't the purpose of homework to be able to come up with arguments on your own? Even though - and pretty much the entire class - perfectly understand the arguments we find online, we probably wouldn't be able to come up with the tricks ourselves. Is this a productive way of doing math? If so, why? If not, what is the alternative approach, considering that pretty much the entire class uses these external resources and probably cannot come up with the more elaborate arguments on its own?
Doing homework is a learning process and it could be done through text books, library books or websites.
Students' job is learning and teacher's job is providing guidance and help to ease the learning process.
If the instructor allows using the internet on homework then so be it.
I would not give more than $20%$% weight to homework any way and we can assign homework problems that require research so students have to use resources available to them.