Taxes. If anything the SIs make more use of the freelancers than the people buying the services in the first place. It's common for a team from a supplier to be augmented by a handful of subcontracted freelancers.
This has become way more common in the last 12 months in the UK public sector as the government has demanded that departments verify the employment status and tax arrangements of their freelance contractors. Effectively they've gone and hidden within the big boys, in return for their taking a 10% cut off the top.
This is the government saying now the big boys have to do the same verification as well.
Of course all the freelancers' tax affairs are entirely above board so they've got nothing to worry about.