I have to agree with Dave. Liability was one of my primary reasons for being so uptight about the use of silica bb's. I've seen BB Bastards, Airsoft Elites, Madbull, etc., shatter or split in half on occasion when striking metal surfaces. I used to run an angle grinder in a steel fabrication shop and even with a full safety shield and glasses I still received a couple slivers of steel in the eye. Silica was much too big of a risk for me and a much larger risk than plastic fragments from cheap bb's. I've always considered cheap bb's more of a risk to the guns than to the players, they jam mags and guns and break internals and half the time they don't even fly straight. I've always been of the opinion that if someone chooses to abuse their gear by using crap bb's it's their own problem.
That being said, if the host wants to limit the players to better quality bb's it's within his right to do so, but I would suggest having some quality bb's available for sale to new players. Unfortunately regardless of this, even with waivers and everyone agreeing to abide by the rules the host could still be found liable if something goes wrong and someone gets injured. Waivers don't offer much protection in court.