Upvote:1
The answer is several fold:
1) Most investigation was done after the fact.
2) It was never expected not to miss some agents in the background check drag nets if you will.
3) The possible strategic information gain from defectors is greater than letting some agents through. Given security at intelligence agencies, the aforementioned agents wouldn't be able to infiltrate without much more in depth vetting anyway.
I could go on.
4) Addendum: You potentially make yourself more sympathetic to possible allies and you claim moral high ground domestically and diplomatically if you allow refugees. I'm sure some allowances were made for these reasons.
Upvote:2
The simple answer: they did not. I personally know many Soviet refugees. The only cases when they had to undergo any security screening were the cases when they applied for sensitive jobs, like in CIA or NSA.