Seems odd, but I am not sure if it is rude, depending on what the invitation was like.
In my area, you get an invitation to the wedding ceremony, and then usually included in it is a separate card inviting you to the reception. There is also usually a reception response card where you can say "yes we're coming" or "no we can't" and sometimes it also has a spot to choose your meal, if you can attend.
Truthfully, if I didn't get the reception card or the response card, I would know that I was not invited to the reception. Then it would be my decision as to whether or not I wanted to go to the wedding ceremony.