Michael Bailey notes in his Magic and Superstition in Europe: A Consice History from Antiquity to the Present that "Perhaps because divinatory practices were so central to the Roman public cults, superstitio gradually came to imply all forms of false or non-Roman rites. It could be applied to foreign cults, which Roman authorities typically tolerated, although they also feared that these cults would somehow sap the virtue of Roman citizens. The cult of the Egyptian goddess Isis was particularly popular in Rome around this time, for example, and was consequently also often held up for particular censure."

This suggests that there was a certain level of popularity that was acceptable for a foreign cult, and that reaching past that could invite censure from the state. As Rome is not my area of study (I only remember this anecdote because it specifically deals with an Egyptian goddess), I will not elaborate further. Hopefully someone else will.