This restriction is actually there for security purposes to prevent PTB-based composition attacks. According to the documentation, there's no 'UnsafeRandom' object available for use to work around these constraints. You might have to consider developing your own custom Pseudorandom Number Generator (PRNG) if you need more flexibility.

The restriction is intended and thoroughly explained in the SUI documentation here: https://docs.sui.io/guides/developer/advanced/randomness-onchain#programmable-transaction-block-ptb-restrictions. It's done to ensure safety and integrity within the system.