We are talking about millions of dishes. This is where law ridicules itself and impossible to enforce. There are some police raids once a while, but not effective.
But not useful for their original purpose (in this case, suppressing satellite TV usage). Just useful as general leverage against targeted people, which the Iranian government is not short of anyway.
The actual purpose is not to supress satellite TV usage but to keep the population in sufficient fear of powers-that-be. You can do many things, just don't stick your head out.