What is Illegal on sending them to sanctuary cities ?
I was wondering the same thing. A quick Internet search, did not produce clarity on this issue. The whole idea of sanctuary cities is very likely misunderstood. A sanctuary city's municipal laws tend to protect undocumented immigrants from deportation or prosecution, despite federal immigration law.
What this means is local authorities do not share information they collect about undocumented immigrants with USCIS. These folks can still be arrested for laws they break or crimes they commit by the local police. For example, if an undocumented immigrant is stopped for a traffic violation and the officer discovers they are an illegal immigrant, they still have to go through the appropriate process, such as paying a fine or in the case of a DUII, be jailed.
It seems to me when a immigrant seeking asylum is moved to a sanctuary city to await their hearing, USCIS already knows they are undocumented. What they won't know is if they commit any crimes while staying in a sanctuary city.
In my opinion, this is a massive hoax Trump is playing on the public with the intent of keeping folks riled up and fearing undocumented immigrants.