It's not about safety. It's about stopping people to get together and have great time, without permission. the statement of free parties is - "we don't need your permissions, your 'safety', your rules etc. To have good time together with excellent sound." Or something along those lines. I've been to maybe 20 raves like that and I can tell you that people are usually very organized and mindful of what is going on, strangers all become friends and it creates feeling of beeing part of big family. - so everyone takes care of each other, leaving prejudice outside and have come together to enjoy the sound. i've seen an illegal rave, on private land somewhere in France, came around 1000 people, 2 stages with massive sound systems. For two days people were dancing to high quality bass music. The best part was that after 2 days, the music stopped and organizers asked attendants to please pick up the trash before leaving. After 2h you could not find single cigarette butt in the grass. Everyone took responsibility and did their part. Only damage that was done was stomped grass.

Edit: this you don't see in parties with entrance fee, because people automatically assume that they have paid to the cleaners, so they shit all they want. Where in free party, people understand that they are the organizers, only thing set up by someone, was wall of sound. The rest is up to the whole collective.

Fire hazards are super rare, usually warehouses have multiple exits. and drugs are being taken the same as in legal parties, like in clubs in the heart of the city. So in my opinion, this attack on free parties is only to stop people from getting together on their own free terms, and feeling united by sound and nothing else.