People sharing their "political" views on a college campus is not that unusual. Unfornately, some of these views can be quite "extreme" or use more offensive methods to get their "message" across. Throwing away this unsolicited material is the best policy.
I would tell your daughter it is okay to say no to accepting such materials. I don't believe the "escort" should have showed the material to your daughter if it was so awful. She is too young at this point. If she continues to pursue her goal in becoming a vet, she will have her own experiences in this area soon enough. I applaud her determination. She may be instrumental in treating animals that have been subject to cruelty.
I would turn this unfornate "incident" into a positive. Use it as an oportunity to discuss both your views on such subjects. Get to know how your daughter feels about it. There may be more contraversial handouts in the future. Especially, safe sex, anti-drugs, drinking, and tobacco brochures. It is a college campus and these are things that discussed on campus on a regular basis. This may be a good time to open up a "dialogue". 12 years old now a days is more mature than when we were 12!