Opposition research can, of course, be a campaign violation, when it's a thing of value provided by a foreign government to assist a campaign.
Think of it this way: Would it be illegal if the Clinton campaign had purchased hundreds of reams of paper from a Venezuelan-government-run business, at market prices, to print campaign literature on? No, it wouldn't, since in that case it was just an ordinary market transaction, and the national source of the commodity makes no difference. But what if the Venezuelan government had given the Clinton campaign hundreds of thousands of dollars of paper free of charge, in order to help her win the campaign? Well, now it's a campaign finance violation, since that's functionally no different than if they'd just handed her the money and she later bought the paper from elsewhere.
In the same sense, if the Trump team paid someone to do opposition research, at free market prices, that wouldn't be a campaign finance violation, regardless of the nationality of the worker. But if the Russian government DONATED that valuable opposition research to the Trump campaign, in order to help him win his campaign, that's a campaign violation. That's what the Trump campaign was conspiring to do: to meet secretly with agents of the Putin regime in order to solicit valuable donations of opposition information to the campaign, as part of Russia's efforts to get Trump elected. That was a campaign violation. And it was collusion.
