Most likely, swastika cakes are not a service that any bakeries (Jewish or otherwise) offer to anybody. The issue here is a minority getting the same service already offered to everybody else.
Conscience must trump greed if we are to remain human. Although there may be no specific prohibition of the creation of pro-Nazi images in any canonical Jewish text as there might be against a pork sandwich (there might be some tolerance for ironic use, as in some movies by Mel Brooks, who is Jewish), I can imagine just about everyone doing a double-take. I would and I am not Jewish.
Company policy may prohibit offensive images, the use of profane words, and (most likely) violations of copyright laws. There might be a prohibition in a bakery against the creation of birthday cakes with Disney characters because the bakery has not gotten permission to use such images -- and not because of any other offense.
If I owned a bakery I would preclude pornographic images, overt profanity, statements of potential for political offense (the hammer-and-sickle or a burning cross would be in the same category as Nazi $#!+), religious affront, or (most likely to create problems) violations of copyright laws.
... There are people who consider working on the Sabbath or dealing in such commodities as pornography, liquor, tobacco, or firearms violations of their morals... but such personal restrictions largely precludes one from some low-paying jobs in retailing.
