Why would a mailman ask not once but 3 times If I was running a business when I shipped today?

I had 33 media mail packages {about normal for me} and one parcel post to a friend, as well as one priority mail flat rate envalope to get a free pickup. Maybe 4 were half.com sales - jsut to cover shipping on the PBS ones.

Is there a reason for this?

That probably has to do with the 'Carrier Connect' program. The carrier has a USPS sales rep contact the customer to explain why shipping by USPS is better than UPS or FedEx. It's a way to 'grow' the business for USPS.

Might just be curiosity.  All the clerks at my post office love to chat with me.  I think they get bored.

Next time a clerk asks, ask them why they are asking (nicely).  Bring them a PBS card and invite them to be members.

Well I'm sure their station probably gets awards, bonus' etc.., for upping sales. If you said yet maybe he would have tried to sell you a postage scale or postage machine or business services. 

I think some post offices have business only lines.  If you are mailing more than 10 packages, you could be considered a business and need to make an appt to send your books or something like that.