I was thinking about this, and I'm not sure what the rule is.
On the one hand you have 'Meccano', 'Playmobile' and similar, when referring to collections of brand items.
But on the other hand, you would refer to a single brand car park as 'Toyotas' or a collection of single brand laptops as 'Lenovos' without anyone batting an eye.
I'm with you - 'legos' sounds stupid to me - but really I need someone more formally trained in grammar for this one :)