Because everything is illegal in california, including piranhas and freshwater rays. I know, it sucks.
"I went to Petco and bought my breeder mice today... The guy gave me a hard time, though, when I asked for a male & female. He said "we're not supposed to sell them for breeding - or as feeders" (despite the fact that I've bought feeders there before ). So I replied "Oh, I meant to say they're pets, and I have another cage at home." He rolled his eyes and got them for me, LOL. They're set up in a little cage now, with a water bottle, food, a chew toy, wheel, a sleeping (strawberry) hut, and a toilet paper roll. Now we'll see if they decide to do the bom-chicka-wow-wow..."
^ Yeah, I hate how petco's policy is like that. Anyway I'm glad you managed to buy them, and good luck with breeding them.