This isn't actually true, as a
general rule. "Campaigns may not solicit or accept contributions from foreign nationals. Federal law prohibits contributions, donations, expenditures and disbursements solicited, directed, received or made directly or indirectly by or from foreign nationals in connection with any election — federal, state or local."
Notice that being about "expenditures?" Basically, campaigns can't do any business, whether donations or buying something (expenditure) with a foreign business or individual. As such, any meetings with a foreign business is potentially evidence of "collusion" (or to use the legal term, conspiracy) -- there is no reason for a campaign to have any "meetings" with a foreign business -- and many campaign laws that support the idea a campaign should not have meetings with foreign businesses.
Now to be clear, a campaign going and buying a Samsung TV at Best Buy for their campaign headquarters is legal -- they are dealing with an American company, despite buying a foreign manufactured television from the American Company. If the campaign dealt with Samsung corporate (which is in South Korea) to buy the TV, that would be a violation of the law.
To give a real world example, one I find Republicans get confused about; the Clinton campaign (through their lawyers) hired Fusion GPS to do opposition research on Donald Trump. Fusion GPS then hired people who investigated Trump in other countries he has done business in, which includes Russia. Much like the TV, it does not matter that the company that the campaign hired to do the work then did business with foreign business or individuals -- much like buying a Samsung TV from Best Buy is not a violation. It could have been a violation if the campaign had dealt directly with these foreign individuals/businesses to get "dirt" on Trump.
To get an idea of why: the thought is to prevent foreign interference in US elections (such as a campaign being "given" (gifts or even reduced prices) merchandise/information, etc -- as that is essentially a "campaign contribution." Worse, it can lead to "favors" -- where the campaign is helped where the candidate is "obligated" to then do favors for these foreign entities. So, we ban these types of meetings/transactions, as a general rule, to help safeguard our elections and prevent foreign influence in both elections and the government.
So the meeting with Russians at Trump tower -- based on the above FEC laws -- was "evidence" of conspiracy (collusion since you keep wanting to use that word). Now, it was not necessarily illegal -- though if any information had been given it becomes a very gray area (since they'd be receiving "something of value" from a foreign source). What largely saved them, in the Mueller investigation, is that no information appears to have been given -- as you point out, the Russians wanted to talk about the
Magnitsky Act, which led to Russia, in response, to banning adoptions (why those in the Trump campaign kept saying it was "about adoption" -- that sounds better than claiming these Russians were trying to get sanctions removed).
In the email, which came from a "Russian individual," it clearly stated, "that I schedule a meeting with you and The Russian government attorney." It was very clear that the meeting would be with Russian "agents" (a Russian Government attorney). Perhaps it was only "Russian individuals" that showed up for the meeting, since working for the Russian government did not appear to be their primary jobs -- but based on the emails, they were likely there representing the Russian government.
And this is just one piece of evidence. You also had the Manafort, as the campaign chairman, giving confidential campaign polling information to a "Russian individual" (but one with ties to, and believed to be working for the Russian government in this capacity). You have George Papadopoulos, drinking with an Australian diplomat, bragging about contacts with the Russian government and how they have hacked emails from Hillary. Then there is Roger Stone who was in communication with Julian Assange (again, not American) and Wikileaks. There was plenty of evidence of possible conspiracy.
