Why do I get telemarketing calls only when my computer is on?

It's just a coincidence. Telemarketers have a specific time of day that they call which is based upon what target audience they are aiming for. These people do research on what time of day certain groups of people are most likely to be home. Most adults work during the day so if they want to sell a man a fancy new razor they aren't going to call his home at 10 in the morning since he isn't likely to be there. But he is more likely to be there at 6PM. It just depends on many things - gender target audience, age of the person, etc, - and when these groups may most likely be home - so you might get a morning call/afternoon call/ etc. But most adults are home after 5PM and weekends, especially Sundays, so this is why most telemarketing occurs during these time periods.

So they are calling when you are on your computer not because you are on your computer, but because you just happen to be on your computer at the time of day when they typically call.

The 2 are not related, purely coincidental.

Coincidence your usage time occurs during thier business hour