A student posed these questions to me. I would like somebody's input to confirm that what I told him was correct. Here goes:

This sentence is correct:
We have our son attend class every Saturday.

But, this sentence is also correct:
New designs have people buying many cars this Fall.

Why is the verb in the first sentence, 'attend,' subjuncted, but not in the second 'buying.' In each case, 'have' is acting as a causitive verb, so why?

Also, are there similar patterns with other causative verbs, such as make, allow, etc.?

Any input on this would satisfy my curiosity.