Being a CPCU, I contacted one of the Kansas CPCU Chapters and asked them this question. They responded that it is not illegal to require renters insurance in Kansas and is a best practice in risk mitigation. They did state that some insurers in the area may not allow additional insured status, but other insurers will, it's up to the underwriter if they offer AI or not.
They then went on to state they they would recommend putting the requirement in the lease (contract) so you can evict for breach of contract (if you were so inclined).
Easier than eviction is people with multiple properties (usually 25+, so it's great for PM) can get forced place renters coverage, so they have it in the lease that they have to have it, when they don't or they let it lapse, then a fee is contractually added to the monthly rent of $10-20 to pay for the forced-place renters insurance.
It works well this way, because the lease would read that the first $10-20 paid each month without renters insurance goes to the forced placed renters insurance. So if their rent was $500, you let them know their policy expired, we've added the charge, they only pay $500, you then send them a late notice with $20 still due on rent and $50 late fee.
It increases fee income as well as protects you from tenant caused and insurable damages.
A disclaimer to the above, I talked to an insurance person that operates in this sphere all day, but it was not a KS real estate attorney.