You can't really blame someone for wondering about this when it happens so often.
As a foreigner who dated an american in the past I agree it hurts to hear that you are after a status/GC/money when you genuinely care about the person. It is humiliating and wrong.
"It is humiliating and wrong"
While I don't think it's right, I can understand why it might cross a person's mind.
Many cultures have a different idea about marriage. If you are living in a country where there is no future because the economy is horrible and there are no jobs, and all you need to do is marry an American to not only get out of this situation, but also help your family. Wouldn't you do it?
Many choose to do this and because it's so prevalent from some countries, it becomes the rule, not the exception.
"Deceiving" marriages, sham marriages or other 'mail-order' brides services are unfortunately a thing, but to say that it has become the rule for relationships between Americans and non-Americans is a naive shortcut and is certainly untrue.