Nairobi media outlets are now reporting in the last few days that the "awareness phase" of the plastic bag ban is over and that arrests have begun of some engaged in manufacturing plastic bags surreptitiously at night.

As others here have said, there's no indication that they're as yet going after individuals, let alone tourists.

However, this was reported in late August:

“Since duty free shops at airports are considered to be outside the Kenyan territory, bags used at this point are not affected by the ban. However, any traveller coming into Kenya with duty free bags shall be required to leave the same at the entry points,” the authority’s director general Geoffrey Wahungu said in a notice to all manufactures, importers and users of plastic bags.
https://www.the-star.co.ke/news/2017...rport_c1624249

As others have noted, above, enforcement even of that seems to be spotty.