going direct to ARIN/RIPE/APNIC etc have some stricter rules (IIRC, in RIPE you need a company or possibly business, plus a bank account in that name). by going for somone like HostUs, they are sponsoring the account, so they need to verify you and ask questions, then setup the ASN, etc. Also, going direct will provide you with V4/V6 space allocations. They might charge a yearly fee (IIRC, its like 50 per allocation) but v6 is "practally" free but V4 will be a in a wait list... Finally, going direct, there is some sort of "kick back"... if the RIR makes more money than it cost to run the place, they reallocate fees back to the LIRs... so, you some times get money back... I have been tempted, but for RIPE, its 1400 per year, plus 2k setup... ouch...
[EDIT] bit more on the RIPE fees: https://www.ripe.net/participate/member-support/payment/ripe...