How come I can block spam on Thunderbird on my iMac but not on my iPad email?

You've already answered your question...

I know there's no spam filter on the iPad.

There are filters on the iMac Mail applications but not on the iPad. To block or filter spam on the iPad requires that you use a spam filter provided by your email provider. My email providers use Red Condor and Barracuda. They seem to work ok.