What 433mhz modules? Low powered ones like SparkFun sells?
There is a limit to unlicensed use for the 70cm band. This doc sums it up
Basically, it has to be under 1 milliwatt and periodic use or something like that. Also the device has to have an FCC Part 15 sticker on it. If it doesn't then it didn't pass FCC inspection. Anything above you need to have a ham license. Just to note, it is easy to get a technician level ham license. It's a $15 test. Find a local club that does classes.
433 is getting more popular. 433 is used in r/c aircraft for long distance control and telemetry. With the recent popularity in multirotors you are seeing more and more people get 433 equipment without realizing that they may need a license for it.
433 is also an ISM band in Europe and Africa - so definitely stay away from that if you are in that region.