only the EU stands up to major monopolies operating on a world scale
And the US government (for those following the Safe Harbor debate).
To take just one example of the cloud-cuckoo land the leave camp inhabit, lets look at the EU directive on personal data - protecting it from being shipped wholesale outside the EU (including the US). (www.theregister.co.uk/2016/06/07/germany_fines_adobe_data_privacy/)
So the UK does leave the EU - trebles all round and we can start the mass transfer of UK citizens data (well what the US hasn't already got) to Uncle Sam. However, the EU rules about their citizens data will still apply. So no business based in the EU will be allowed to do any business with the UK unless the UK provides a guarantee that it will protect that data.
Of course, you could imagine that outside the EU, the UK would have better protection for personal data.
And you'd be wrong - (viz. the current Communications Data Bill)
So now we have a UK outside the EU, but unable to business with any EU country unless it follows EU data protection - for EU citizens. But UK citizens ? Who cares ? Not the EU. Nor the UK government either.
And that's one example.
Oh, and no trade treaty in the world would be able to bypass the EUs data protection requirements.
I suspect one reason why US companies want a remain is it means they won't have to relocate out of the UK. Something they would be required to consider if the UK cannot offer the guarantees the EU would require of a trading partner.
Sorry to have managed to write so much without a mention of immigration.