Given that cat repellent products such as this (not a recommendation, just knew they existed and this is the first one I found to link to) are available in the US and marketed as such, one must conclude that they are legal to sell for this use. Same goes for other types of products such as sprays. Note that you will not find an air rifle marketed as a cat repellent (on any remotely reputable site).
Even American animal welfare charities offer advice on how to achieve this, and again one would assume that they have checked out the advice they are offering is both legal and in the animals' best interests.
The Animal Legal Defence Fund offers what looks like a detailed description of animal protection laws and how they vary by state. I don't think it is feasible to summarise into this answer, but for instance (and of course, I Am Not A Lawyer, etc), it seems that in New York if you knowingly used something poisonous to cats, you would be
... guilty of a misdemeanor, punishable by imprisonment for not more than one year, or by a fine of not more than one thousand dollars, or by both.
(Not likely, given your aim is to keep that cats away from something poisonous to them).