Is it true that there is a policy to not send probes anywhere that conditions might support life?
To my knowledge, there is no policy that explicitly prohibits sending a probe to potentially habitable extraterrestrial environments. Mars is a prime example, and probes have been sent there for decades now. It will be something similar with Europa, for which a mission was recently approved.
In fact, scientists are very interested in sending probes to those places. They want to try to answer the question of whether there is life elsewhere in the solar system. Right now, the best bet is to land a robotic probe where you suspect there could be life, and have the probe perform exploration and analysis of planetary materials right there. Some people argue that, in the long run, the most efficient way to find living organisms outside Earth will be to send astronauts.
There is, however, an international policy of “planetary protection” followed by the largest space agencies, like NASA. NASA has an Office of Planetary Protection, one of whose objectives is to promote the avoidance of biological contamination of extraterrestrial environments (for example, Mars). Planetary protection is in itself a fascinating scientific field, and research to preserve solar system objects in their natural state is ongoing.
The link to the NASA Office of Planetary Protection is http://planetaryprotection.nasa.gov/
I hope this helps!
Prof. Augusto Carballido